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 changes and test-gated benchmarks; no proving, auth, or transaction-handling logic changes.

Overview
Adds structured validation warnings in virtual_snos_prover so each early reject logs event = "validation_error" with a stable reason (pending_block_unsupported, non_invoke_transaction, invalid_transaction_input) via inspect_err, without logging transaction calldata.

Updates the RPC layer’s prove failure log to event = "prove_transaction_failed" with outcome from metric_outcome(), matching prover_prove_transaction_outcome_total for log/metric correlation.

Introduces an ignored in-test microbenchmark (server::overhead_bench) comparing a no-layer baseline to HttpMetricsLayer steady-state per-request overhead (50k ops, release + --ignored).

^{Reviewed by Cursor Bugbot for commit eba2906. 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: 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: 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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