This commit brings the URL mode elicitation implementation into full
compliance with the SEP 1036 specification as defined in PR modelcontextprotocol#887 of the
specification repository.

Changes include:

1. Added elicitation completion notifications
   - New ElicitCompleteNotification type and parameters
   - send_elicit_complete() method in ServerSession
   - Handler in ClientSession for processing completion notifications
   - Servers can now notify clients when URL mode elicitations complete

2. Corrected error code to match specification
   - Changed from -32000 to -32042 per spec
   - Renamed ELICITATION_REQUIRED to URL_ELICITATION_REQUIRED
   - Error code now in MCP-specific range [-32000, -32099]

3. Added comprehensive test coverage
   - Test for completion notification sending and receiving
   - Test for correct error code value
   - All tests passing (13/13 elicitation tests)

The implementation now supports all aspects of SEP 1036:
- URL mode elicitation requests with proper parameters
- Completion notifications for async workflows
- Correct error handling with spec-compliant error codes
- Full backward compatibility with form mode elicitation

Github-Issue:modelcontextprotocol#1036