starknet_transaction_prover: add --log-format text|json CLI flag

[avi-starkware]

starknet_transaction_prover: add --log-format text|json CLI flag

Adds a LogFormat enum and --log-format / LOG_FORMAT=text|json
selector that switches the tracing subscriber between the text and JSON
formatters. Default is text. Matches the discovery-service convention.

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

[cursor]

PR Summary

Low Risk
Observability-only change to log formatting at startup; no proving, RPC, or security behavior is modified.

Overview
Adds a --log-format / LOG_FORMAT switch (text | json, default text) on the Starknet transaction prover so operators can emit structured JSON logs for aggregators without changing the existing RUST_LOG filter defaults.

Startup now parses CLI args before initializing tracing_subscriber, then picks either the plain text or .json() fmt layer from the new LogFormat enum. Config tests were updated so synthetic CliArgs include the default text format.

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

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[einat-starkware]

@einat-starkware reviewed 3 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on avi-starkware).