Add MCP 2026 cacheable result hints

[leehack]

Summary

• add optional cacheable result fields (ttlMs/cacheScope) to tools, prompts, resources, resource templates, and resource reads
• inject 2026 stateless server defaults for resultType, ttlMs, and cacheScope on cacheable operations while preserving legacy result JSON when hints are unset
• cover explicit hints, legacy default serialization, negative/invalid cache metadata handling, and stateless server wire defaults

Spec context

• MCP draft caching requires cache hints on tools/list, prompts/list, resources/list, resources/templates/list, and resources/read
• 2026 stateless result examples use resultType: complete for completed results

Validation

• dart format .
• dart analyze
• dart test test/mcp_2026_07_28_test.dart
• dart test test/mcp_2026_07_28_test.dart test/mcp_2025_11_25_test.dart test/types/resources_test.dart test/types_test.dart test/tool_schema_test.dart test/client/client_test.dart
• dart test -j 1
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 98.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.16%. Comparing base (c9a341c) to head (5af9b80).

Files with missing lines	Patch %	Lines
lib/src/types/json_rpc.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                            Coverage Diff                            @@
##           spec/2026-task-extension-capabilities     #186      +/-   ##
=========================================================================
+ Coverage                                  86.00%   86.16%   +0.16%     
=========================================================================
  Files                                         46       46              
  Lines                                       8294     8362      +68     
=========================================================================
+ Hits                                        7133     7205      +72     
+ Misses                                      1161     1157       -4     

Flag	Coverage Δ
unittests	86.16% <98.88%> (+0.16%)	⬆️

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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.