Fix ReadyForQuery queue stall with explicit transactions

[SeanTAllen]

The query state machine only transitioned to _QueryReady when the ReadyForQuery status byte was 'I' (idle). Non-idle statuses ('T' for in-transaction, 'E' for failed transaction) left the state machine in _QueryNotReady, permanently stalling the queue. This made explicit transactions (BEGIN/COMMIT/ROLLBACK) unusable.

ReadyForQuery means the server is ready for the next command regardless of the status byte — the byte is informational, not a readiness gate. Remove the msg.idle() checks from all five query states and always transition to _QueryReady. Also remove the msg parameter from _QueryState.on_ready_for_query since no query state inspects it.

Two new integration tests exercise the fix: Transaction/Commit (exercises status 'T') and Transaction/RollbackAfterFailure (exercises status 'E').

Design: #102

[ponylang-main]

Hi @SeanTAllen,

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