Consolidate MCP 2026 feature foundation

[leehack]

Summary

• consolidate the initial MCP 2026 RC discovery, stateless HTTP, tool parameter header, MRTR, subscription, and task extension wire work
• preserve the existing dual-stack behavior: 2026 is the branch default while legacy protocol support remains available
• replaces the first part of the prior micro-PR stack (Add MCP 2026 RC discovery foundation #178-Enforce MCP 2026 task extension capabilities #185)

Validation

• dart format --output=none --set-exit-if-changed .
• dart analyze
• git diff --check
• dart test
• dart pub global run coverage:test_with_coverage

This is a draft RC PR and should stay off main until the official spec release.

[codecov]

Codecov Report

❌ Patch coverage is 93.45898% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.00%. Comparing base (9a5ea35) to head (7348727).

Files with missing lines	Patch %	Lines
lib/src/server/streamable_https.dart	88.32%	16 Missing ⚠️
lib/src/server/server.dart	90.90%	11 Missing ⚠️
lib/src/types/tasks.dart	91.83%	8 Missing ⚠️
lib/src/types/json_rpc.dart	93.51%	7 Missing ⚠️
lib/src/client/client.dart	95.76%	5 Missing ⚠️
lib/src/types/subscriptions.dart	94.11%	5 Missing ⚠️
lib/src/server/streamable_mcp_server.dart	91.83%	4 Missing ⚠️
lib/src/server/mcp_server.dart	86.36%	3 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           dev/2026-07-28-rc     #267      +/-   ##
=====================================================
+ Coverage              81.11%   86.00%   +4.88%     
=====================================================
  Files                     67       46      -21     
  Lines                  10090     8294    -1796     
=====================================================
- Hits                    8185     7133    -1052     
+ Misses                  1905     1161     -744     

Flag	Coverage Δ
cli	?
unittests	86.00% <93.45%> (+1.08%)	⬆️

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