perf: reduce allocations & reflection in generated operation code

[sergey-tihon]

Summary

Reduces allocations and reflection in both the OperationCompiler (compile-time) and the code generated for every API operation (runtime). Validated end-to-end against the full Stripe spec3.json (414 paths, 1422 schemas, 587 operations).

Compile-time (OperationCompiler / Provider.OpenApiClient)

• Hoist RuntimeHelpers.toParam / RuntimeHelpers.toQueryParams / TaskExtensions.cast / AsyncExtensions.cast MethodInfo resolution to the OperationCompiler instance — resolved once per provider instead of per generated operation.
• Hoist string list union-case reflection in Provider.OpenApiClient.
• Build a paramName -> IOpenApiParameter map once per operation and use it for O(1) lookups in invokeCode (was Seq.tryFind per ShapeVar).
• Build the fixed Content-Type / Accept headers list at provider-generation time instead of emitting a quotation that re-evaluates per call.
• Make response quotations (object / stream / string / unit) lazy so only the branch actually used for the operation's return type is built.

Generated code (runtime)

• Emit RuntimeHelpers.createHttpRequestFromQueryLists instead of chained List.append / List.concat for query parameter assembly.
• Emit RuntimeHelpers.fillHeadersAndCookies instead of an inline Seq.filter / Seq.map / String.concat cookie-building quotation.
• For typed JSON responses, emit a direct generic TaskExtensions.cast<'T> / AsyncExtensions.cast<'T> call via ProvidedTypeBuilder.MakeGenericMethod, avoiding MethodInfo.Invoke on every API call.
• Avoid the previous shared-quotation Var hazard that caused FS3033 "An item with the same key has already been added" for operations with multiple path/query parameters.

Runtime helpers (RuntimeHelpers)

• Add createHttpRequestFromQueryLists: flattens seq<#seq<string*string>> into a ResizeArray, filtering nulls, and delegates to existing createHttpRequest.
• Add fillHeadersAndCookies: calls fillHeaders, then builds the Cookie header imperatively with StringBuilder using the standards-compliant "; " separator and TryAddWithoutValidation.

Tests

• Add regression tests in Schema.OperationCompilationTests that assert generated invokeCode does not reuse the same Quotations.Var instance across path/query parameters (would otherwise cause FS3033 in real providers).

Behavior changes

• Cookie header separator changed from ";" to "; " (RFC 6265 canonical form). Servers accept both; flagged for visibility.

Validation

• dotnet fantomas --check src/ tests/ ✅
• Unit tests: 479 total, 0 failed, 1 skipped ✅
• Provider integration tests (Swashbuckle server on localhost:5000): 127 total, 0 failed ✅
• Stripe spec3.json (414 paths, 1422 schemas, 587 operations) compiles successfully.
• Generated-code inspection on Stripe:
  • reflective casts: 0
  • generated List.append calls: 0
  • generated List.concat calls: 0
  • duplicate quotation Var bindings: 0

[Copilot]

Pull request overview

This PR optimizes OpenAPI operation generation by reducing repeated reflection and simplifying generated runtime request/response code.

Changes:

• Hoists method/union-case reflection and parameter lookup setup into provider/operation compilation paths.
• Adds runtime helpers for query-list flattening and combined header/cookie filling.
• Adds regression tests for duplicate quotation Var reuse in generated invoke code.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/SwaggerProvider.DesignTime/OperationCompiler.fs	Optimizes generated operation code and response casting.
src/SwaggerProvider.DesignTime/Provider.OpenApiClient.fs	Hoists string-list union-case lookup.
src/SwaggerProvider.Runtime/RuntimeHelpers.fs	Adds query-list and header/cookie runtime helpers.
tests/SwaggerProvider.Tests/Schema.OperationCompilationTests.fs	Adds quotation Var reuse regression tests.

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