Consolidate MCP 2026 result semantics

[leehack]

Summary

• consolidate structured tool output, MRTR input-required prompt/resource results, resource-not-found behavior, MCP metadata keys, cache TTL, input responses, and elicitation result validation
• isolates cross-feature result semantics from lower-level transport enforcement
• replaces the result/input semantics portion of the prior micro-PR stack (Allow MRTR input-required prompt and resource results #209-Validate capability object entries #218)

Validation

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

Stacked on #268. This is a draft RC PR and should stay off main until the official spec release.

[codecov]

Codecov Report

❌ Patch coverage is 95.18717% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.11%. Comparing base (f3542a8) to head (e9b94c7).

Files with missing lines	Patch %	Lines
lib/src/server/server.dart	93.50%	5 Missing ⚠️
lib/src/types/validation.dart	83.33%	3 Missing ⚠️
lib/src/server/mcp_server.dart	96.96%	1 Missing ⚠️

Additional details and impacted files

@@                         Coverage Diff                         @@
##           spec/2026-transport-enforcement     #269      +/-   ##
===================================================================
- Coverage                            87.94%   84.11%   -3.83%     
===================================================================
  Files                                   47       69      +22     
  Lines                                 8957    11790    +2833     
===================================================================
+ Hits                                  7877     9917    +2040     
- Misses                                1080     1873     +793     

Flag	Coverage Δ
cli	70.55% <ø> (?)
unittests	88.08% <95.18%> (+0.14%)	⬆️

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