Bug fix: a missing type variable ReceiveResultT in the BaseSession class definition.

[huangjia2019]

Bug fix: a missing type variable ReceiveResultT in the BaseSession class definition.

Motivation and Context

Added ReceiveResultT as a generic type parameter to BaseSession.

This aligns BaseSession's generic signature with its actual method usage, particularly for:

send_request() which returns a ReceiveResultT

_send_response() which also sends a ReceiveResultT in some cases

Why
Without ReceiveResultT in the class signature, type checking tools (like Pyright or MyPy) may raise warnings or fail to infer correct types.

The class was already using ReceiveResultT internally, so this fix improves type completeness and consistency.

Impact
Improves type safety and developer experience for consumers of the MCP Python SDK

How Has This Been Tested?

No behavior change; purely type annotation fix

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

[huangjia2019]

Will someone review this PR?