feat: add MCP Apps helper APIs and examples

[leehack]

Summary

• add first-class MCP Apps support with typed helpers and constants, including TypeScript-style server APIs (registerAppTool, registerAppResource, getUiCapability) and _meta.ui compatibility handling
• preserve MCP Apps metadata across resources/resource contents and add list-level _meta support for registered resources/templates
• add docs, examples, and tests for MCP Apps flows, and update CLI inspect to gracefully skip unsupported capability methods while advertising MCP Apps extension support

Testing

• dart analyze
• dart test
• cd packages/mcp_dart_cli && dart analyze && dart test

[codecov]

Codecov Report

❌ Patch coverage is 90.44944% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.93%. Comparing base (e5e4945) to head (1ed28a7).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/src/types/mcp_ui.dart	89.43%	13 Missing ⚠️
lib/src/server/mcp_server.dart	25.00%	3 Missing ⚠️
lib/src/server/mcp_ui.dart	97.14%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
+ Coverage   78.49%   78.93%   +0.43%     
==========================================
  Files          40       42       +2     
  Lines        5381     5559     +178     
==========================================
+ Hits         4224     4388     +164     
- Misses       1157     1171      +14     

Flag	Coverage Δ
unittests	78.93% <90.44%> (+0.43%)	⬆️

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