Align MRTR input response result shapes

[leehack]

Summary

• serialize MRTR ElicitResult and ListRootsResult input responses using the MCP 2026 embedded client-result shapes without common Result _meta
• preserve CreateMessageResult sampling metadata where the draft schema still permits message _meta
• validate raw/wire MRTR input responses so deprecated client-result _meta is rejected in 2026 inputResponses

Validation

• dart format .
• dart analyze
• git diff --check
• dart test test/mcp_2026_07_28_test.dart --name "serializes MRTR retry fields on supported client requests|rejects malformed MRTR wire shapes"
• dart test test/mcp_2026_07_28_test.dart
• dart test
• (cd packages/mcp_dart_cli && dart run mcp_dart_cli:mcp_dart conformance --suite all --json)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.38%. Comparing base (271147f) to head (f1564e1).

Additional details and impacted files

@@                         Coverage Diff                          @@
##           spec/stateless-http-cors-headers     #231      +/-   ##
====================================================================
+ Coverage                             88.36%   88.38%   +0.01%     
====================================================================
  Files                                    47       47              
  Lines                                  9294     9308      +14     
====================================================================
+ Hits                                   8213     8227      +14     
  Misses                                 1081     1081              

Flag	Coverage Δ
unittests	88.38% <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.