fix(jet): fail health checks when transaction enqueue closes

[ananthb]

Summary

• replace expect("transaction sink closed") with a typed TransactionPipelineUnavailable error on all transaction enqueue paths
• add transaction_pipeline_health to Jet readiness so /health returns 503 when the local scheduler enqueue pipeline is closed
• return HTTP 503 for /api/v1/transactions when the enqueue pipeline is unavailable instead of treating it as a bad request
• add a regression test for a valid transaction submitted after the scheduler receiver is dropped

Context

This addresses the Jet-side failure mode behind abrupt backend closes before HAProxy receives response headers. The enqueue failure means Jet cannot hand accepted transactions to its local scheduler; that should make the alloc fail readiness so Consul can stop routing to it until restart/recovery recreates the pipeline.

No Linear ticket — incident follow-up.

Tests

• cargo test --package yellowstone-jet