starknet_transaction_prover: origin-level breadcrumbs for validation errors

[avi-starkware]

Splits validation-error logging so each origin (request body, RPC,
proving) carries an event tag and trimmed payload context, making it
straightforward to root-cause production failures from the structured
log stream. Also adds an in-test microbenchmark for HttpMetricsLayer
overhead so any regression in the hot path is visible.

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

[cursor]

PR Summary

Low Risk
Observability-only logging and test-gated benchmarks; no proving, auth, or request-handling behavior changes.

Overview
Adds structured validation and failure logging so production issues can be traced without logging private transaction calldata.

In virtual_snos_prover, each validation failure now emits a warn with event = "validation_error" and a stable reason (pending_block_unsupported, non_invoke_transaction, invalid_transaction_input) before returning. The RPC layer’s prove failure log is updated to use event = "prove_transaction_failed" and outcome from metric_outcome(), aligned with prover_prove_transaction_outcome_total.

Also adds server/overhead_bench, ignored release microbenchmarks that compare baseline Tower throughput vs HttpMetricsLayer steady-state overhead for operators reproducing observability rollout numbers.

^{Reviewed by Cursor Bugbot for commit 894c08e. Bugbot is set up for automated code reviews on this repo. Configure here.}

[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 👈 (View in Graphite)
• 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,tower_ohttp: OHTTP-unlinkable request-id for decapsulated content #14222
• 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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[reviewable-StarkWare]

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