Add TS 2026 RC interop smoke

[leehack]

Summary

• Add a manual TypeScript SDK 2026 RC interop fixture pinned to the upstream PR #2327 preview package.
• Cover modern 2026-07-28 negotiation, tools/list, and tools/call from the TS preview client to the Dart 2026 RC conformance server.
• Mark server/discover as a cacheable 2026 result so stateless responses include ttlMs and cacheScope hints.
• Update the CLI conformance harness fixtures to return cache hints for mock server/discover responses while keeping CLI lower-bound compatibility with the published SDK dev package.
• Update interoperability and 2026 RC release docs, including the current conformance alpha.4 package reference.

Why

The published/current TypeScript SDK packages do not yet provide a clean 2026 RC interop target. The PR #2327 preview includes the modern Streamable HTTP header behavior needed for a real client-to-server smoke. That smoke exposed a Dart-side server/discover cache-hint gap, which this PR fixes with focused regression tests and matching CLI conformance fixtures.

Validation

• dart run tool/testing/run_ts_2026_rc_interop.dart
• dart run bin/mcp_dart.dart conformance --suite all --json from packages/mcp_dart_cli
• dart pub global run pana --no-warning from packages/mcp_dart_cli (160/160)
• dart analyze
• dart test test/mcp_2026_07_28_test.dart
• dart test test/server/streamable_mcp_server_test.dart
• dart test
• dart format --output=none --set-exit-if-changed .
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.91%. Comparing base (ad0b8e9) to head (84134a2).

Additional details and impacted files

@@                Coverage Diff                 @@
##           dev/2026-07-28-rc     #286   +/-   ##
==================================================
  Coverage              84.90%   84.91%           
==================================================
  Files                     70       70           
  Lines                  15383    15390    +7     
==================================================
+ Hits                   13061    13068    +7     
  Misses                  2322     2322           

Flag	Coverage Δ
cli	76.72% <ø> (-0.01%)	⬇️
unittests	88.45% <100.00%> (+<0.01%)	⬆️

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