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 change to proving logic, auth, or transaction handling beyond log fields.

Overview
Proving validation now emits structured warn logs at each early-return path (event = "validation_error" with distinct reason values for pending blocks, non-invoke txs, and invalid input), without logging transaction calldata.

The RPC layer replaces a debug-style failure log with structured fields (event = "prove_transaction_failed", outcome aligned with prover_prove_transaction_outcome_total) as a catch-all when proving fails.

Adds ignored in-test microbenchmarks (server::overhead_bench) to compare baseline Tower throughput vs HttpMetricsLayer steady-state overhead for operators reproducing observability rollout numbers.

^{Reviewed by Cursor Bugbot for commit cd33cbf. 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: unify HTTP middleware stack via macro #14286
• main-v0.14.3

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[reviewable-StarkWare]

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