feat: Add protocol_version parameter to ClientSession

[goingforstudying-ctrl]

Summary

Fixes #2307 - Adds the ability to specify a custom protocol version in ClientSession.initialize().

Problem

ClientSession.initialize() hardcodes protocolVersion=types.LATEST_PROTOCOL_VERSION with no way for the caller to override it. When connecting to an MCP server that requires a specific protocol version (e.g., Snowflake's managed MCP server requires "2025-06-18"), the only workaround is to mutate the module-level constant before calling initialize().

Solution

Added an optional protocol_version parameter to ClientSession.init():

def __init__(
    self,
    ...
    protocol_version: str | None = None,  # NEW — defaults to LATEST_PROTOCOL_VERSION
    *,
    ...
) -> None:
    self._protocol_version = protocol_version or types.LATEST_PROTOCOL_VERSION

Then use it in initialize():

protocolVersion=self._protocol_version,

Changes

File	Change	Reason
session.py	Added protocol_version parameter	Allow custom protocol version
session.py	Store as _protocol_version instance var	Use in initialize()
session.py	Update initialize() to use self._protocol_version	Use custom version
test_session.py	Added test_client_session_custom_protocol_version	Verify custom version works
test_session.py	Added test_client_session_default_protocol_version	Verify backwards compatibility

Testing

• Added test for custom protocol version
• Added test for default protocol version (backwards compatibility)
• Both tests pass

Backwards Compatibility

Fully backwards compatible - existing callers get the current default (LATEST_PROTOCOL_VERSION) when protocol_version is not specified.

Closes #2307