starknet_transaction_prover: /health returns 503 when service is saturated

[avi-starkware]

Adds SaturationMonitor (shared by ProvingRpcServerImpl and
HealthLayer) that tracks whether the concurrency semaphore has been
continuously rejecting proving requests. Once that has held for the
configured window (health_max_saturated_ms, default 10s), /health
returns 503 with an opaque body so load balancers can drain the pod
before in-flight requests start failing.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

[cursor]

PR Summary

Medium Risk
Changes load-balancer health signaling and pod draining behavior; mis-tuned thresholds could drain healthy pods or leave overloaded ones in rotation, though defaults and recovery on accept limit blast radius.

Overview
Adds saturation-aware /health so load balancers can drain pods when the prover stays at its concurrency limit.

A shared SaturationMonitor records when prove_transaction hits the semaphore (mark_rejected) and clears on a successful acquire (mark_accepted). HealthLayer uses that state: after health_max_saturated_ms (default 10s) of continuous rejections, GET /health returns 503 with an opaque {"status":"unhealthy","reason":"saturated"} body; a successful accept restores 200. Wiring runs through main, ProvingRpcServerImpl, and HTTP/HTTPS start_server / start_tls_server, with HEALTH_MAX_SATURATED_MS / config file support. Unit tests cover the monitor and health behavior.

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[avi-starkware]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• starknet_transaction_prover: document observability surface in README #14203
• starknet_transaction_prover: origin-level breadcrumbs for validation errors #14172
• starknet_transaction_prover: /health returns 503 when service is saturated #14171 👈 (View in Graphite)
• starknet_transaction_prover: bump prover_panics_total on panic hook fire #14170
• starknet_transaction_prover: HTTP request count + latency + in-flight metrics #14169
• starknet_transaction_prover: proving-job duration + outcome metrics #14168
• starknet_transaction_prover: Prometheus /metrics endpoint with build_info #14167
• starknet_transaction_prover: global panic hook + graceful SIGTERM shutdown #14166
• starknet_transaction_prover: per-request structured log with request-id #14165
• starknet_transaction_prover: redact URL credentials in logs and add startup banner #14164
• starknet_transaction_prover: add /health endpoint via HealthLayer #14163
• starknet_transaction_prover: add --log-format text|json CLI flag #14162
• main

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