docs: add missing docstrings to core client/server session API methods

[sohammmmm10]

Add docstrings to undocumented public methods in the client and server session modules:

• ClientSession.initialize(): document handshake flow, return type, and RuntimeError on unsupported protocol version
• ServerSession.client_params: document property returning client's InitializeRequestParams
• ServerSession.incoming_messages: document the ServerRequestResponder stream property
• remove_request_params(): document URL query-stripping utility

These are the most user-facing public methods that currently lack docstrings, making IDE tooltips and help() output incomplete.

Motivation and Context

How Has This Been Tested?

Breaking Changes

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

[sohammmmm10]

Note: The single failing check (CI / test — 3.12, windows-latest) is a
pre-existing coverage gap (99.96% vs 100% threshold) unrelated to this PR.
This PR adds docstrings only — zero logic changes. The required all-green
check passes, and all 26 other checks are green.

Happy to rebase if needed once this is reviewed.