Consolidate JSON-RPC metadata hardening

[leehack]

Summary

• consolidate JSON value, metadata object, stateless result/header, request identity, progress token, JSON-RPC envelope, and nested metadata validation hardening
• keeps protocol-envelope compatibility and stateless metadata routing reviewable in one layer
• replaces the JSON-RPC/metadata hardening portion of the prior micro-PR stack (Validate URL elicitation URIs #219-Prefer nested metadata for stateless routing #246)

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

[codecov]

Codecov Report

❌ Patch coverage is 93.26047% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.47%. Comparing base (e9b94c7) to head (c994327).

Files with missing lines	Patch %	Lines
...kages/mcp_dart_cli/lib/src/conformance_runner.dart	85.14%	15 Missing ⚠️
lib/src/types/sampling.dart	80.59%	13 Missing ⚠️
lib/src/types/initialization.dart	95.38%	3 Missing ⚠️
lib/src/server/streamable_mcp_server.dart	91.30%	2 Missing ⚠️
lib/src/types/content.dart	95.12%	2 Missing ⚠️
lib/src/client/streamable_https.dart	93.33%	1 Missing ⚠️
lib/src/types/tasks.dart	95.23%	1 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                       @@
##           spec/2026-result-semantics     #270      +/-   ##
==============================================================
+ Coverage                       84.11%   84.47%   +0.36%     
==============================================================
  Files                              69       69              
  Lines                           11790    12098     +308     
==============================================================
+ Hits                             9917    10220     +303     
- Misses                           1873     1878       +5     

Flag	Coverage Δ
cli	71.04% <85.14%> (+0.49%)	⬆️
unittests	88.45% <95.08%> (+0.36%)	⬆️

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