fix: use explicit skill invocation for A2A tool calls

Implement explicit skill invocation (deterministic) by default for A2A
protocol, as recommended by the A2A specification. This uses DataPart
with skill name and parameters instead of natural language interpretation.

**Changes:**
- Add `use_explicit_skill` parameter to `_call_a2a_tool` (default: True)
- Explicit mode: Send DataPart with `{"skill": "...", "parameters": {...}}`
- Natural language mode: Send TextPart with formatted text (legacy)
- Import DataPart from a2a.types

**Benefits:**
- Works without authentication for discovery endpoints (test agent)
- Predictable, repeatable behavior
- Direct skill execution without AI interpretation overhead
- Aligns with A2A best practices

**Reference:**
https://docs.adcontextprotocol.org/docs/protocols/a2a-guide

**Testing:**
- ✅ list_authorized_properties works without auth
- ✅ list_creative_formats works without auth
- ✅ get_products validates parameters correctly

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

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

Author: bokelley

src/adcp/protocols/a2a.py

@@ -9,7 +9,16 @@
 
 import httpx
 from a2a.client import A2ACardResolver, A2AClient
-from a2a.types import Message, MessageSendParams, Part, Role, SendMessageRequest, Task, TextPart
+from a2a.types import (
+    DataPart,
+    Message,
+    MessageSendParams,
+    Part,
+    Role,
+    SendMessageRequest,
+    Task,
+    TextPart,
+)
 
 from adcp.exceptions import (
     ADCPAuthenticationError,
@@ -115,19 +124,50 @@ async def close(self) -> None:
             self._httpx_client = None
             self._a2a_client = None
 
-    async def _call_a2a_tool(self, tool_name: str, params: dict[str, Any]) -> TaskResult[Any]:
-        """Call a tool using A2A protocol via official a2a-sdk client."""
+    async def _call_a2a_tool(
+        self, tool_name: str, params: dict[str, Any], use_explicit_skill: bool = True
+    ) -> TaskResult[Any]:
+        """
+        Call a tool using A2A protocol via official a2a-sdk client.
+
+        Args:
+            tool_name: Name of the skill/tool to invoke
+            params: Parameters to pass to the skill
+            use_explicit_skill: If True, use explicit skill invocation (deterministic).
+                               If False, use natural language (flexible).
+
+        The default is explicit skill invocation for predictable, repeatable behavior.
+        See: https://docs.adcontextprotocol.org/docs/protocols/a2a-guide
+        """
         start_time = time.time() if self.agent_config.debug else None
         a2a_client = await self._get_a2a_client()
 
         # Build A2A message
         message_id = str(uuid4())
-        text_part = TextPart(text=self._format_tool_request(tool_name, params))
-        message = Message(
-            message_id=message_id,
-            role=Role.user,
-            parts=[Part(root=text_part)],
-        )
+
+        if use_explicit_skill:
+            # Explicit skill invocation (deterministic)
+            # Use DataPart with skill name and parameters
+            data_part = DataPart(
+                data={
+                    "skill": tool_name,
+                    "parameters": params,
+                }
+            )
+            message = Message(
+                message_id=message_id,
+                role=Role.user,
+                parts=[Part(root=data_part)],
+            )
+        else:
+            # Natural language invocation (flexible)
+            # Agent interprets intent from text
+            text_part = TextPart(text=self._format_tool_request(tool_name, params))
+            message = Message(
+                message_id=message_id,
+                role=Role.user,
+                parts=[Part(root=text_part)],
+            )
 
         # Build request params
         params_obj = MessageSendParams(message=message)