Allow JSON-value structured tool output

[leehack]

Summary

• allow MCP 2026 tool outputSchema to use any JSON Schema while preserving object-root inputSchema enforcement
• allow CallToolResult and sampling tool_result structuredContent to carry any JSON value, including explicit null
• keep stable 2025 callers compatible by omitting non-object output schemas and non-object structuredContent from stable server responses

Validation

• dart format .
• dart analyze
• dart test test/tool_schema_test.dart test/types/sampling_test.dart test/client/client_tool_validation_test.dart test/server/output_validation_test.dart
• dart test
• git diff --check
• (packages/mcp_dart_cli) dart analyze
• (packages/mcp_dart_cli) dart test
• (packages/mcp_dart_cli) dart run bin/mcp_dart.dart conformance --suite all --json

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.95%. Comparing base (e867775) to head (9245893).

Files with missing lines	Patch %	Lines
lib/src/types/validation.dart	81.25%	3 Missing ⚠️
lib/src/server/mcp_server.dart	95.83%	1 Missing ⚠️

Additional details and impacted files

@@                            Coverage Diff                            @@
##           spec/2026-client-subscriptions-listen     #208      +/-   ##
=========================================================================
+ Coverage                                  87.94%   87.95%   +0.01%     
=========================================================================
  Files                                         47       47              
  Lines                                       8957     9002      +45     
=========================================================================
+ Hits                                        7877     7918      +41     
- Misses                                      1080     1084       +4     

Flag	Coverage Δ
unittests	87.95% <92.85%> (+0.01%)	⬆️

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[leehack]

Closing this draft micro PR because the RC spec work has been consolidated into a smaller reviewable stack. This change is preserved in #268 (MCP 2026 transport enforcement); the consolidated tip is tree-identical to the previous micro-stack tip.