fix: address latest transport handshake review feedback

[Spicy-cream]

Summary

Address the latest review feedback on PR3535.

Changes

• Restore the _cq_sid guard before removing an RDMA completion-channel consumer.
• Return TRY_OTHERS for prefixes that cannot match UBSHM or RDMA handshake magic.
• Preserve NOT_ENOUGH_DATA for valid fragmented UB, RDMA, and RDM3 prefixes.
• Fall back to TCP when RDMA or UBSHM endpoint allocation fails instead of failing the socket.
• Correct UBSHM polling flag descriptions.
• Restore four-space indentation in the affected transport and endpoint code.
• Add regression tests for impossible and fragmented handshake magic prefixes.

Validation

• Build: passed.
• Common transport handshake tests: 19/19 passed.
• UBSHM/UBRING tests: 15/15 passed.
• RDMA tests: 86/86 passed.

This PR targets the source branch used by apache#3535.