Consolidate type schema validation and conformance gates

[leehack]

Summary

• consolidate remaining MCP 2026 RC type/schema validation hardening for discovery results, elicitation primitives, icons, implementation fields, integer fields, URI/base64 fields, annotations, roles, enum values, request wrappers, and content/result unions
• add dedicated official conformance fixtures and runners for stable MCP 2025-11-25 and MCP 2026 RC client/server coverage
• wire official 2025 and 2026 conformance gates into core CI, with expected-failure files for intentional RC conformance churn
• preserve object-level JSON Schema 2020-12 keywords on JsonObject parse/serialize round-trips so official json-schema-2020-12 coverage stays wire-compatible
• replaces the final validation portion of the prior micro-PR stack (Require nested metadata for stateless HTTP #247-Validate enum wire fields #266)

Reviewer Map

• Type and schema validation: lib/src/types/*, lib/src/shared/json_schema/*, lib/src/types/elicitation.dart, lib/src/types/json_rpc.dart
• Runtime behavior touched by validation: lib/src/client/*, lib/src/server/*, lib/src/shared/protocol.dart
• Official conformance fixtures/runners: test/conformance/*
• CI/docs/changelog: .github/workflows/test_core.yml, test/conformance/README.md, CHANGELOG.md
• Regression coverage: test/mcp_2025_11_25_test.dart, test/mcp_2026_07_28_test.dart, streamable HTTP/client/server tests, JSON Schema tests

Validation

• dart format --output=none --set-exit-if-changed .
• git diff --check
• dart analyze
• dart test
• dart run test/conformance/run_2025_server_conformance.dart --timeout-seconds 90 --output-dir .dart_tool/conformance/ci_2025_server (49 passed, 0 failed)
• npx -y @modelcontextprotocol/conformance@0.2.0-alpha.1 client --command "dart run test/conformance/mcp_2026_rc_client.dart" --suite all --spec-version 2025-11-25 --verbose -o .dart_tool/conformance/ci_2025_client (242 passed, 0 failed)
• dart run test/conformance/run_2026_rc_server_conformance.dart --timeout-seconds 90 --output-dir .dart_tool/conformance/ci_2026_server (19 passed, 0 failed/timeout)
• dart run test/conformance/run_2026_rc_client_conformance.dart --timeout-seconds 90 --output-dir .dart_tool/conformance/ci_2026_client (35 passed, 0 failed/timeout)

Stacked on #270. This is a draft RC PR and should stay off main until the official spec release.

[codecov]

Codecov Report

❌ Patch coverage is 88.53503% with 234 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.75%. Comparing base (c994327) to head (0c5ca39).
⚠️ Report is 47 commits behind head on dev/2026-07-28-rc.

Files with missing lines	Patch %	Lines
lib/src/client/streamable_https.dart	51.55%	78 Missing ⚠️
lib/src/client/client.dart	78.37%	48 Missing ⚠️
lib/src/shared/protocol.dart	81.81%	24 Missing ⚠️
lib/src/types/json_value.dart	36.36%	21 Missing ⚠️
lib/src/server/server.dart	88.77%	11 Missing ⚠️
lib/src/types/sampling.dart	91.45%	10 Missing ⚠️
lib/src/server/mcp_server.dart	81.39%	8 Missing ⚠️
lib/src/shared/json_schema/json_schema.dart	92.13%	7 Missing ⚠️
lib/src/types/tools.dart	89.85%	7 Missing ⚠️
lib/src/server/streamable_mcp_server.dart	76.19%	5 Missing ⚠️
... and 7 more

Additional details and impacted files

@@                  Coverage Diff                  @@
##           dev/2026-07-28-rc     #271      +/-   ##
=====================================================
+ Coverage              84.47%   84.75%   +0.27%     
=====================================================
  Files                     69       70       +1     
  Lines                  12098    15364    +3266     
=====================================================
+ Hits                   10220    13022    +2802     
- Misses                  1878     2342     +464     

Flag	Coverage Δ
cli	76.73% <100.00%> (+5.68%)	⬆️
unittests	88.24% <88.52%> (-0.21%)	⬇️

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