ServerSession methods (create_message, elicit_form) don't expose progress_callback parameter

[maxisbey]

Summary

The ServerSession high-level methods for sending requests to clients (create_message, elicit_form) don't expose the progress_callback parameter, even though the underlying BaseSession.send_request() fully supports it.

This means servers can't easily receive progress notifications from clients during sampling or elicitation requests.

Current Behavior

# ServerSession.elicit_form() - no progress_callback parameter
async def elicit_form(
    self,
    message: str,
    requestedSchema: types.ElicitRequestedSchema,
    related_request_id: types.RequestId | None = None,
) -> types.ElicitResult:
    return await self.send_request(...)  # progress_callback not passed through

# ServerSession.create_message() - same issue
async def create_message(
    self,
    messages: list[types.SamplingMessage],
    *,
    max_tokens: int,
    # ... other params ...
    related_request_id: types.RequestId | None = None,
) -> types.CreateMessageResult:
    return await self.send_request(...)  # progress_callback not passed through

Expected Behavior

# Should be able to pass progress_callback
result = await server_session.elicit_form(
    message="Please provide your details",
    requestedSchema=schema,
    progress_callback=lambda progress, total, msg: print(f"Progress: {progress}/{total} - {msg}")
)

result = await server_session.create_message(
    messages=messages,
    max_tokens=1000,
    progress_callback=lambda progress, total, msg: print(f"Sampling progress: {progress}/{total}")
)

Context

• The MCP spec supports bidirectional progress notifications - clients CAN send notifications/progress back to servers during request handling
• BaseSession.send_request() already supports progress_callback parameter
• ClientSession.call_tool() exposes progress_callback for the client→server direction
• The TypeScript SDK exposes this via RequestOptions.onprogress in both createMessage() and elicitInput()
• Tests in tests/shared/test_progress_notifications.py demonstrate the bidirectional flow works

Suggested Fix

Add progress_callback: ProgressFnT | None = None parameter to:

• ServerSession.create_message()
• ServerSession.elicit_form()
• Any other ServerSession methods that send requests to clients

And pass it through to send_request().

[BryceEWatson]

I'd like to take this on — PR incoming.

I've added progress_callback: ProgressFnT | None = None to create_message(), elicit_form(), elicit_url(), and the deprecated elicit() wrapper on ServerSession, plus threaded it through the elicitation.py helpers and Context API. The approach mirrors how ClientSession.call_tool() already handles it.

Includes E2E tests for both sampling and elicitation flows. All existing tests pass.