starknet_transaction_prover: add /health endpoint via HealthLayer

[avi-starkware]

Adds a tower middleware that short-circuits GET /health ahead of
jsonrpsee (which would 405 a GET) and returns {"status":"ok"}. Wired
outermost in both the HTTP and HTTPS middleware chains so load balancers
can probe before any per-request work runs.

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

[cursor]

PR Summary

Low Risk
Read-only liveness endpoint with no auth, data, or RPC behavior changes beyond skipping middleware for GET /health.

Overview
Adds a Tower HealthLayer that answers GET /health with 200 and {"status":"ok"} before jsonrpsee (which would reject GET with 405). The layer is registered outermost on both plain HTTP and HTTPS middleware stacks so probes skip CORS, OHTTP, compression, and RPC handling.

Exports HEALTH_PATH and HealthLayer from server. Unit tests cover the happy path, non-GET on /health, and GET on other paths (all fall through to the inner service).

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[reviewable-StarkWare]

This change is 

[avi-starkware]

• 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
• 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 👈 (View in Graphite)
• starknet_transaction_prover: add --log-format text|json CLI flag #14162
• main

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[avi-starkware]

@avi-starkware resolved 1 discussion.
Reviewable status: 0 of 4 files reviewed, all discussions resolved.

[einat-starkware]

@einat-starkware reviewed 4 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on avi-starkware).