feat(macros): auto-generate get_info and default router

[DaleSeo]

Motivation and Context

Close #711

Building a minimal tool-only MCP server now requires just three pieces of boilerplate beyond the actual tool logic. You need a tool_router: ToolRouter<Self> field in the struct, a constructor to initialize it with Self::tool_router(), and a get_info() override to enable the tool's capability since the trait default doesn't enable anything.

This PR updates the three handler macros to automatically generate get_info() with the correct capabilities if the user doesn't provide one. The default router for #[tool_handler] and #[prompt_handler] has changed from field access to calling the associated function. This change removes the need for a stored router field and its initialization boilerplate.

Now, you can write a minimal tool server like this:

struct TimeServer;

#[tool_router]
impl TimeServer {
    #[tool(description = "Get current time")]
    async fn get_time(&self) -> String { "12:00".into() }
}

#[tool_handler]
impl ServerHandler for TimeServer {}

How Has This Been Tested?

New integration tests cover the field-free server pattern, custom name/version/instructions attributes, manual get_info() preservation, and an end-to-end tool call over duplex transport.

Breaking Changes

No breaking changes to the public API. The auto-generated get_info() is only added when the user does not provide one, so existing implementations are unaffected.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context