Escape MCP parameter header sentinels

[leehack]

Summary

• base64-escape literal Mcp-Param-* values that match the =?base64?...?= sentinel pattern
• add client coverage for encoded sentinel-shaped tool arguments
• add server validation coverage for decoded sentinel literals and document the RC behavior

Validation

• dart format .
• dart analyze
• dart test test/client/streamable_https_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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.57%. Comparing base (edb7d6a) to head (8ca6827).

Additional details and impacted files

@@                                Coverage Diff                                 @@
##           spec/2026-no-server-requests-on-stateless-streams     #195   +/-   ##
==================================================================================
  Coverage                                              86.57%   86.57%           
==================================================================================
  Files                                                     46       46           
  Lines                                                   8578     8581    +3     
==================================================================================
+ Hits                                                    7426     7429    +3     
  Misses                                                  1152     1152           

Flag	Coverage Δ
unittests	86.57% <100.00%> (+<0.01%)	⬆️

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.