Fix memory accounting when message reads are cancelled

[ekhodzitsky]

When a backend exits while a client message is only partially received, wait_for_next_message drops the pending read. Its reservation remains in CURRENT_MEMORY, so later clients can receive SQLSTATE 53200 even after the original client disconnects.

Use a scope guard in both message readers to release the reservation on completion, error, or cancellation. Tests cover partial headers and bodies, overlapping reservations, task cancellation, EOF, and memory-limit rejection. The BDD regression confirms that a reservation is active, terminates the backend, and checks that the next client can query within the configured limit.

Validation:

• cargo test --workspace --lib --bins --test message_memory: 1254 passed, 1 ignored.
• BDD @stale-server-detection on PostgreSQL 18.6: 6 scenarios, 130 steps passed.
• Cancellation regressions fail on the original implementation and pass with the fix.