Implement auto completion

[leehack]

This pull request adds explicit support for the Completions capability in accordance with the MCP 2025-06-18 specification. The changes introduce a new ServerCapabilitiesCompletions class, update the server and client logic to use this explicit capability, and provide comprehensive tests and examples to demonstrate and validate the new behavior.

Completions Capability Support:

• Added a new ServerCapabilitiesCompletions class to represent completions-related capabilities, including an optional listChanged field. (lib/src/types.dart lib/src/types.dartR534-R553)
• Updated the ServerCapabilities class to include a new completions field, with appropriate serialization and deserialization logic. (lib/src/types.dart [1] [2]
• Modified the client capability check logic to require the explicit completions capability for completion-related requests, rather than inferring support from prompts or resources. (lib/src/client/client.dart lib/src/client/client.dartL152-R153)

Documentation and Examples:

• Added an example (completions_capability_demo.dart) demonstrating how to declare and use the completions capability, including resource and prompt argument completion. (example/completions_capability_demo.dart example/completions_capability_demo.dartR1-R162)
• Updated the README.md to mention the new Completions capability. (README.md README.mdR27)
• Updated the CHANGELOG.md with details of the new capability and related changes. (CHANGELOG.md CHANGELOG.mdR1-R8)

Testing:

• Added integration tests to verify server capability declarations, serialization, and round-trip behavior for completions. (test/integration/completions_capability_test.dart test/integration/completions_capability_test.dartR1-R105)
• Added unit tests for the new ServerCapabilitiesCompletions class and its integration with ServerCapabilities. (test/types_test.dart test/types_test.dartR71-R105)

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 46.23%. Comparing base (1f08bd5) to head (037fc3d).
⚠️ Report is 15 commits behind head on main.

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
+ Coverage   45.55%   46.23%   +0.68%     
==========================================
  Files          15       16       +1     
  Lines        2880     2911      +31     
==========================================
+ Hits         1312     1346      +34     
+ Misses       1568     1565       -3     

Flag	Coverage Δ
unittests	46.23% <90.00%> (+0.68%)	⬆️

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