refactor: Migrate to datamodel-code-generator for type generation

Replace custom 900-line type generator with battle-tested datamodel-code-generator library.

## Changes

### Type Generation
- Generate Pydantic models from JSON schemas using datamodel-code-generator
- All 101 schemas now generate proper Pydantic v2 models
- Models inherit from AdCPBaseModel to maintain `exclude_none=True` serialization
- Generated types use proper Pydantic types (AnyUrl, AwareDatetime, Field validators)

### Backward Compatibility
- Added 22 type aliases to preserve existing API surface
- Aliases: ActivateSignalSuccess/Error, CreateMediaBuySuccess/Error, etc.
- Deployment/Destination variants: PlatformDeployment, AgentDeployment, etc.

### Schema Sync Improvements
- Added ETag support for efficient schema updates
- Added --force flag to bypass ETags and force re-download
- Cache ETags in .etags.json for conditional downloads
- Better status reporting (not modified, updated, cached, forced)

### Test Updates
- Fixed test_discriminated_unions.py for new schemas
- Fixed test_format_id_validation.py for new error messages
- Updated Error object construction to use dict syntax
- Added URL type handling for AnyUrl comparisons
- All migration-related tests passing (219/229 total)

### Files
- New: src/adcp/types/generated_poc/ (101 generated type files)
- Modified: src/adcp/types/generated.py (consolidated exports)
- Modified: scripts/sync_schemas.py (ETag support)
- Modified: schemas/cache/ (synced with latest AdCP v1.0.0)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: bokelley

schemas/cache/.etags.json

@@ -13,7 +13,7 @@
       "type": "array",
       "description": "Target destination(s) for activation. If the authenticated caller matches one of these destinations, activation keys will be included in the response.",
       "items": {
-        "$ref": "destination.json"
+        "$ref": "/schemas/v1/core/destination.json"
       },
       "minItems": 1
     },

schemas/cache/1.0.0/activate-signal-request.json

@@ -13,7 +13,7 @@
           "type": "array",
           "description": "Array of deployment results for each destination",
           "items": {
-            "$ref": "deployment.json"
+            "$ref": "/schemas/v1/core/deployment.json"
           }
         },
         "context": {
@@ -40,7 +40,7 @@
           "type": "array",
           "description": "Array of errors explaining why activation failed (e.g., platform connectivity issues, signal definition problems, authentication failures)",
           "items": {
-            "$ref": "error.json"
+            "$ref": "/schemas/v1/core/error.json"
           },
           "minItems": 1
         },

schemas/cache/1.0.0/activate-signal-response.json

@@ -54,7 +54,7 @@
       "type": "array",
       "description": "Array of all properties covered by this adagents.json file. Same structure as list_authorized_properties response.",
       "items": {
-        "$ref": "property.json"
+        "$ref": "/schemas/v1/core/property.json"
       },
       "minItems": 1
     },
@@ -119,7 +119,7 @@
             "type": "array",
             "description": "Specific properties this agent is authorized for (alternative to property_ids/property_tags). Mutually exclusive with property_ids and property_tags fields.",
             "items": {
-              "$ref": "property.json"
+              "$ref": "/schemas/v1/core/property.json"
             },
             "minItems": 1
           },

schemas/cache/1.0.0/adagents.json

@@ -5,7 +5,7 @@
   "description": "Brand manifest provided either as an inline object or a URL string pointing to a hosted manifest",
   "oneOf": [
     {
-      "$ref": "brand-manifest.json",
+      "$ref": "/schemas/v1/core/brand-manifest.json",
       "description": "Inline brand manifest object"
     },
     {

schemas/cache/1.0.0/asset-required.json

@@ -10,11 +10,11 @@
       "description": "Natural language instructions for the transformation or generation. For pure generation, this is the creative brief. For transformation, this provides guidance on how to adapt the creative."
     },
     "creative_manifest": {
-      "$ref": "creative-manifest.json",
+      "$ref": "/schemas/v1/core/creative-manifest.json",
       "description": "Creative manifest to transform or generate from. For pure generation, this should include the target format_id and any required input assets (e.g., promoted_offerings for generative formats). For transformation (e.g., resizing, reformatting), this is the complete creative to adapt."
     },
     "target_format_id": {
-      "$ref": "format-id.json",
+      "$ref": "/schemas/v1/core/format-id.json",
       "description": "Format ID to generate. The format definition specifies required input assets and output structure."
     },
     "context": {

schemas/cache/1.0.0/brand-manifest-ref.json

@@ -10,7 +10,7 @@
       "type": "object",
       "properties": {
         "creative_manifest": {
-          "$ref": "creative-manifest.json",
+          "$ref": "/schemas/v1/core/creative-manifest.json",
           "description": "The generated or transformed creative manifest"
         },
         "context": {
@@ -37,7 +37,7 @@
           "type": "array",
           "description": "Array of errors explaining why creative generation failed",
           "items": {
-            "$ref": "error.json"
+            "$ref": "/schemas/v1/core/error.json"
           },
           "minItems": 1
         },

schemas/cache/1.0.0/build-creative-request.json

@@ -13,19 +13,19 @@
       "type": "array",
       "description": "Array of package configurations",
       "items": {
-        "$ref": "package-request.json"
+        "$ref": "/schemas/v1/media-buy/package-request.json"
       }
     },
     "brand_manifest": {
-      "$ref": "brand-manifest-ref.json",
+      "$ref": "/schemas/v1/core/brand-manifest-ref.json",
       "description": "Brand information manifest serving as the namespace and identity for this media buy. Provides brand context, assets, and product catalog. Can be provided inline or as a URL reference to a hosted manifest. Can be cached and reused across multiple requests."
     },
     "po_number": {
       "type": "string",
       "description": "Purchase order number for tracking"
     },
     "start_time": {
-      "$ref": "start-timing.json"
+      "$ref": "/schemas/v1/core/start-timing.json"
     },
     "end_time": {
       "type": "string",
@@ -35,7 +35,7 @@
     "reporting_webhook": {
       "allOf": [
         {
-          "$ref": "push-notification-config.json"
+          "$ref": "/schemas/v1/core/push-notification-config.json"
         },
         {
           "type": "object",

schemas/cache/1.0.0/build-creative-response.json

@@ -70,7 +70,7 @@
           "type": "array",
           "description": "Array of errors explaining why the operation failed",
           "items": {
-            "$ref": "error.json"
+            "$ref": "/schemas/v1/core/error.json"
           },
           "minItems": 1
         },