docs: document session management for streamable HTTP transport

[DaleSeo]

Motivation and Context

The SessionManager trait and the StreamableHttpService are key to how MCP servers manage stateful sessions over Streamable HTTP. However, the current documentation is lacking. There's no overview of the module, no explanation of what a "session" means here, and no examples on how to read the session ID or custom state in a tool handler.

This PR adds rustdoc documentation to close that gap.

How Has This Been Tested?

Documentation-only change. Verified that cargo doc builds without warnings and all existing tests continue to pass.

Breaking Changes

None.

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