Update to new MCP spec version

[gspencergoog]

Description

This updates the dart_mcp package to the latest MCP spec, 2025-06-18.

New Features

• Elicitation: A new elicitation/create request allows the server to ask the client for structured information from the user. This is useful for scenarios where a tool needs more information to proceed. The client can respond with the requested information, or the user can accept, reject, or cancel the request.

• Structured Tool Output: Tools can now declare an outputSchema, and the callTool result can include structuredContent that conforms to this schema. This allows for more robust and predictable tool interactions.

• Resource Links: A new ResourceLink content type can be returned from tools, allowing them to provide links to resources that may not be available through the resources/list request.

API Improvements

• Titles for UI: Many objects in the API now have an optional title field. This provides a human-readable name that can be used in user interfaces, separate from the programmatic name. This has been added to Implementation, Prompt, PromptArgument, Resource, ResourceTemplate, and Tool.

• Completion Context: The completions/complete request now accepts a context object, which can include previously resolved arguments from a URI template or prompt. This allows for more context-aware completions.

• Metadata: A _meta field has been added to many objects to allow for passing arbitrary metadata with requests and responses.

• Last Modified Annotation: A lastModified timestamp can now be included in Annotations to indicate when the data was last updated.

Other Changes

• ResourceReference has been renamed to ResourceTemplateReference for clarity. The old name is preserved as a deprecated typedef.
• The agreed-upon version of the protocol is respected by both the client and server.

Tests

• New tests have been added to cover the new features and API changes.

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Changes to files need to be accounted for in their respective changelogs.

[jakemac53]

LGTM, thanks!