Support nested MCP parameter headers

[leehack]

Summary

• discover x-mcp-header annotations at nested object-property depths in tool schemas
• sync nested mappings into Streamable HTTP transports using JSON Pointer selectors while preserving top-level selector compatibility
• mirror and validate nested tools/call arguments through Mcp-Param-* headers

Validation

• dart format .
• dart analyze
• dart test test/client/client_tool_validation_test.dart
• dart test test/client/streamable_https_test.dart
• dart test test/server/mcp_server_test.dart
• dart test test/server/streamable_https_test.dart
• dart test
• dart pub global run coverage:test_with_coverage
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 98.71795% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.66%. Comparing base (8ca6827) to head (d3f12c2).

Files with missing lines	Patch %	Lines
lib/src/client/client.dart	96.15%	1 Missing ⚠️

Additional details and impacted files

@@                             Coverage Diff                              @@
##           spec/2026-param-header-sentinel-encoding     #196      +/-   ##
============================================================================
+ Coverage                                     86.57%   86.66%   +0.08%     
============================================================================
  Files                                            46       46              
  Lines                                          8581     8631      +50     
============================================================================
+ Hits                                           7429     7480      +51     
+ Misses                                         1152     1151       -1     

Flag	Coverage Δ
unittests	86.66% <98.71%> (+0.08%)	⬆️

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 #268 (MCP 2026 transport enforcement); the consolidated tip is tree-identical to the previous micro-stack tip.