Gate application result fields by protocol version

[KirschBluteX]

Fixes #1754.

Summary

Normalize protocol-version-specific properties on typed results at a single terminal response-emission boundary, after outgoing message filters have run.

• Legacy requests remove resultType and, for cacheable results, ttlMs and cacheScope. The server emits at most one compatibility warning per method, including under concurrent or repeated emissions.
• Requests using protocol version 2026-07-28 or later preserve explicit values and restore required defaults (resultType: "complete", ttlMs: 0, and cacheScope: "private") if a filter removes them.
• Protocol selection uses the request-scoped version and falls back to the negotiated version for stateful sessions.
• The terminal boundary clones and normalizes the post-filter JSON response before transport emission. This covers filter mutation, response replacement, and repeated next calls without mutating shared result instances.
• Normal, cacheable, immediate alternate, and returned/thrown MRTR typed results all pass through the same boundary.
• Raw McpServerOptions.RequestHandlers remain outside this boundary because they operate on already-serialized JSON and intentionally bypass the typed handler infrastructure.

The selected policy is strip plus warning. This keeps legacy wire responses schema-valid while making application misuse visible, following the SDK's existing compatibility-warning convention without turning otherwise valid requests into errors.

Tests

• Full ModelContextProtocol.slnx Release build: 0 warnings, 0 errors.
• Focused protocol-result gating tests: 21 passed.
• Core protocol, MRTR, and message-filter regression set: 45 passed.
• ASP.NET Core non-Manual suite (Execution!=Manual, net10.0): 617 passed, 30 skipped, 0 failed.
• MapMcp and filter-focused tests: 75 passed, 9 skipped, 0 failed.
• git diff --check and targeted dotnet format --verify-no-changes passed.
• Two Docker Everything Server sampling tests were attempted but are not counted as validation: the pinned image returned Unknown tool: trigger-sampling-request before exercising this change.

[KirschBluteX]

@jeffhandley @halter73 Could you review the updated head?

Since 0bf4349, commit 7f47075 moves typed-result normalization after outgoing filters and clones the post-filter response before protocol gating. This prevents filters from reintroducing legacy-incompatible resultType, ttlMs, or cacheScope, while keeping shared result instances unchanged. Regression tests cover normal, cacheable, immediate, and MRTR paths.

Could you confirm this is the intended boundary for #1754 and approve if it looks right?