Prefer nested metadata for stateless routing

[leehack]

Summary

• prefer params._meta over top-level _meta when detecting stateless protocol versions from raw Streamable MCP request bodies
• keep top-level metadata as a fallback for compatibility when nested metadata is absent
• add a transport-level regression for conflicting top-level and nested protocol metadata

Spec notes

• The current stable schema (2025-11-25) defines request metadata under request params.
• The draft 2026-07-28 schema requires request metadata under params._meta for stateless requests.
• This aligns the Streamable MCP routing detector with the JSON-RPC parser precedence from the previous stacked PR.

Sources:

• https://github.com/modelcontextprotocol/modelcontextprotocol/releases
• https://raw.githubusercontent.com/modelcontextprotocol/modelcontextprotocol/main/schema/2025-11-25/schema.json
• https://raw.githubusercontent.com/modelcontextprotocol/modelcontextprotocol/main/schema/draft/schema.json

Verification

• dart format lib/src/server/streamable_mcp_server.dart test/server/streamable_mcp_server_test.dart
• dart test test/server/streamable_mcp_server_test.dart
• dart analyze
• dart test
• (cd packages/mcp_dart_cli && dart analyze && dart test)
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.47%. Comparing base (463167d) to head (2cefe79).

Additional details and impacted files

@@                         Coverage Diff                         @@
##           spec/prefer-nested-request-meta     #246      +/-   ##
===================================================================
+ Coverage                            88.45%   88.47%   +0.02%     
===================================================================
  Files                                   47       47              
  Lines                                 9335     9335              
===================================================================
+ Hits                                  8257     8259       +2     
+ Misses                                1078     1076       -2     

Flag	Coverage Δ
unittests	88.47% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[leehack]

Closing this draft micro PR because the RC spec work has been consolidated into a smaller reviewable stack. This change is preserved in #270 (JSON-RPC metadata hardening); the consolidated tip is tree-identical to the previous micro-stack tip.