Require metadata on server discover requests

[leehack]

Summary

• require server/discover requests to carry nested params._meta request metadata when parsing or serializing
• reject top-level-only or missing metadata for the 2026 discover request shape
• add a CLI conformance case for the server/discover request metadata requirement

Verification

• dart format lib/src/types/initialization.dart test/mcp_2026_07_28_test.dart packages/mcp_dart_cli/lib/src/conformance_runner.dart packages/mcp_dart_cli/test/src/conformance_command_test.dart
• dart test test/mcp_2026_07_28_test.dart
• (cd packages/mcp_dart_cli && dart test test/src/conformance_command_test.dart)
• (cd packages/mcp_dart_cli && dart run mcp_dart_cli:mcp_dart conformance --suite all --json)
• dart analyze
• git diff --check
• (cd packages/mcp_dart_cli && dart analyze)
• dart test
• (cd packages/mcp_dart_cli && dart test)

Spec note: the current draft schema requires server/discover to include params, and common request params require _meta. This remains draft-stack work while 2026-07-28 is RC.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.45%. Comparing base (126d245) to head (f13ede6).

Additional details and impacted files

@@                              Coverage Diff                              @@
##           spec/json-rpc-integer-ids-progress-tokens     #244      +/-   ##
=============================================================================
+ Coverage                                      88.43%   88.45%   +0.01%     
=============================================================================
  Files                                             47       47              
  Lines                                           9325     9338      +13     
=============================================================================
+ Hits                                            8247     8260      +13     
  Misses                                          1078     1078              

Flag	Coverage Δ
unittests	88.45% <100.00%> (+0.01%)	⬆️

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[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.