Gate application result fields by protocol version

[KirschBluteX]

Fixes #1754.

Summary

Add a single typed response-emission boundary after handlers and filters for protocol-version-specific result properties.

• Legacy requests strip resultType and, for cacheable results, ttlMs and cacheScope, and emit a warning when application or filter code supplied them.
• 2026-07-28 and later requests preserve explicit values and add the required defaults (complete, 0, and private).
• The boundary edits freshly serialized JSON nodes, so shared result instances are not mutated across requests or protocol versions.
• Normal, cacheable, immediate alternate, and returned/thrown MRTR result paths retain their type metadata through emission.
• Raw McpServerOptions.RequestHandlers are intentionally outside this typed boundary; they return already-serialized JSON and the existing API contract defines them as bypassing typed handler infrastructure.

The policy is strip plus warning: it keeps legacy wire responses schema-valid and makes application misuse visible, following the SDK's existing compatibility-warning convention without converting otherwise valid requests into errors.

Tests

• Full ModelContextProtocol.slnx Release build: 0 warnings, 0 errors.
• Core protocol-result wire-shape tests: 8 passed.
• Core MRTR emission/backcompat tests: 3 passed.
• ASP.NET Core focused HTTP wire test: 1 passed.
• Core non-Docker suite (Execution!=Manual, net10.0): 2,360 passed, 4 skipped, 0 failed.
• ASP.NET Core non-Docker suite (Execution!=Manual, net10.0): 616 passed, 30 skipped, 0 failed.
• git diff --check and targeted dotnet format --verify-no-changes passed for all changed files.
• Repository-wide format verification still reports pre-existing whitespace diagnostics in untouched files; those unrelated files were not changed.
• Docker Everything Server connection test passed. The unmodified sampling test still calls the removed trigger-sampling-request tool and fails before the SDK sampling handler; the current v1 image exposes sampleLLM. A diagnostic rerun with only that tool name updated passed both Docker tests (2/2); no unrelated fixture change is included here.
• GitHub Actions are currently action_required pending maintainer approval; they were not rerun.

The branch is rebased onto upstream main at 8754e19 (the stateless protocol-span fix); that change does not alter the result-emission boundary.