feat: support progress_callback propagation in ClientSession (issue #1248)

[nkaushal99]

closes #1248

Motivation and Context

This change introduces support for injecting a default progress_handler into ClientSession. It solves the issue where downstream libraries (e.g., LangChain's MCP adapter) access the raw client.session and make tool calls that bypass the top-level fastmcp.Client’s progress_handler, leading to missing progress reports. With this enhancement, consistent progress reporting is ensured whether tools are called through the high-level client or the lower-level session.

How Has This Been Tested?

This feature has been tested within an async environment using:

• Direct calls to ClientSession.call_tool(...) to ensure the default handler is triggered.
• Calls with explicitly provided progress_callback to ensure override behavior works correctly.
• Integration with langchain_mcp_adapters to confirm downstream compatibility and improved reporting.

Breaking Changes

No breaking changes. This enhancement is backward-compatible. Existing users who do not pass a progress_handler will see no change in behavior. Users who want consistent progress reporting can optionally provide a handler to ClientSession.

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

This feature enables clean integration with ecosystem libraries like LangChain without requiring redundant progress callback plumbing at every call site.

[nkaushal99]

link to issue #1248