fix: improve type safety and null handling in protocol methods

[sujalsalekar]

Type Safety and Null Handling Improvements,
Changes,
This PR improves type safety and null handling across several critical protocol-related methods:

BasicHost.get_mux()

Added proper return type annotation (Multiselect),
Implemented null checks for multiselect instance,
Added runtime error when multiselect is not initialized,
Improved method documentation with return types and error cases,

Multiselect.get_protocols()

Changed return type to tuple[TProtocol, ...],
Added filtering of None values from protocol handlers,
Improved type safety by ensuring only valid protocols are returned,

RelayDiscovery._check_via_mux()

Added proper return type annotation (bool | None),
Improved protocol comparison logic with type safety,
Added null checks for host.get_mux(),
Enhanced error handling and documentation,

identify._mk_identify_protobuf()

Added proper error handling for missing host information,
Improved type safety in protocol string conversion,
Added runtime error for missing required data,

Testing,
The changes have been tested with existing test suites. No new tests were required as these changes enforce stricter type checking at the existing code paths.

Impact,
These changes improve the robustness of the codebase by:
Preventing potential null pointer exceptions,
Making type checking more strict,
Improving error messages for debugging,
Maintaining backward compatibility while adding safety,