Align MRTR sampling input request shape

[leehack]

Summary

• omit legacy task metadata from embedded MRTR sampling/createMessage input requests
• reject incoming embedded sampling/createMessage params that still carry legacy task metadata
• add 2026 RC regression coverage for serialization and malformed MRTR parsing

Spec context

• The draft CreateMessageRequestParams schema includes sampling fields such as messages, maxTokens, includeContext, metadata, modelPreferences, toolChoice, and tools, but no legacy task member: https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/draft/schema.json
• This preserves CreateMessageRequest.toJson() for legacy JSON-RPC sampling requests while making the embedded MRTR shape match the 2026 core schema.

Part of #177.

Tests

• dart format lib/src/types/json_rpc.dart test/mcp_2026_07_28_test.dart
• dart test test/mcp_2026_07_28_test.dart
• git diff --check
• dart analyze
• dart test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.48%. Comparing base (15a5d14) to head (9bcb972).

Additional details and impacted files

@@                        Coverage Diff                         @@
##           spec/integer-schema-number-bounds     #251   +/-   ##
==================================================================
  Coverage                              88.48%   88.48%           
==================================================================
  Files                                     47       47           
  Lines                                   9353     9354    +1     
==================================================================
+ Hits                                    8276     8277    +1     
  Misses                                  1077     1077           

Flag	Coverage Δ
unittests	88.48% <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 #271 (type/schema validation); the consolidated tip is tree-identical to the previous micro-stack tip.