Reject negative MCP cache TTLs

[leehack]

Summary

• reject negative cacheable-result ttlMs values while parsing wire JSON
• keep constructor/serialization validation aligned with the same minimum-0 rule
• update 2026 RC regression coverage and changelog

Tests

• dart format .
• dart analyze
• dart test test/mcp_2026_07_28_test.dart
• git diff --check
• dart test test/client/streamable_https_advanced_test.dart -p vm --plain-name "Streamable HTTPS Advanced Integration invalid resumed SSE message reports parser errors"
• dart test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.02%. Comparing base (8630ba2) to head (3e5ef4a).

Additional details and impacted files

@@                      Coverage Diff                       @@
##           spec/2026-meta-key-validation     #212   +/-   ##
==============================================================
  Coverage                          88.02%   88.02%           
==============================================================
  Files                                 47       47           
  Lines                               9066     9067    +1     
==============================================================
+ Hits                                7980     7981    +1     
  Misses                              1086     1086           

Flag	Coverage Δ
unittests	88.02% <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 #269 (MCP 2026 result semantics); the consolidated tip is tree-identical to the previous micro-stack tip.