starknet_transaction_prover: make RPC error spec test exhaustive via enum

[avi-starkware]

Replaces the ad-hoc test_cases vec in test_error_responses_match_spec with a
ServiceErrorCode enum whose spec_key and sample_error_object matches are
exhaustive: adding a variant fails to compile until both are updated, and the
bidirectional set-equality check between the enum and the spec's
components/errors keys fails until the OpenRPC spec documents it.

Each error's code and canonical message now live in one place, with the
constructor functions as thin wrappers.

strum is a dev-dependency and EnumIter is derived under #[cfg(test)]: the
iteration exists only for the test, so it does not belong in the shipped
binary's dependency graph.

Drops the pre-existing TODO(Avi): Add an error enum to make this test exhaustive.

[reviewable-StarkWare]

This change is 

[avi-starkware]

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• starknet_transaction_prover: cover the CORS layer attachment in start_server #15036
• starknet_transaction_prover: unit-test both overload-rejection paths #15035
• starknet_transaction_prover: unit-test the JSON-RPC error mapping #15034
• starknet_transaction_prover: add deployment smoke script and guide #14052
• starknet_transaction_prover: add manual testing guide #14404
• starknet_transaction_prover: drop stale dead_code allow on RpcVirtualBlockExecutor #14740
• starknet_transaction_prover: extract shared server test utils #14739
• starknet_transaction_prover: make RPC error spec test exhaustive via enum #14051 👈 (View in Graphite)
• starknet_transaction_prover: add TLS integration tests #14050
• starknet_transaction_prover: add ServiceConfig validation tests #14049
• starknet_transaction_prover: add CORS validation unit tests #14048
• main-v0.14.4

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[cursor]

PR Summary

Low Risk
Refactors error construction and test wiring without changing RPC behavior; risk is limited to accidental code/message drift, which the new exhaustive checks are meant to catch.

Overview
Introduces ServiceErrorCode so each proving-service JSON-RPC error has a single source of truth for its code and canonical message; existing helpers like block_not_found() and service_busy() now only build ErrorObjectOwned through the enum.

The OpenRPC conformance test test_error_responses_match_spec no longer uses a hand-maintained list. It walks all enum variants (via strum::EnumIter, enabled only under cfg(test)), checks each against the spec through exhaustive spec_key / sample_error_object matches, and asserts set equality between enum keys and components/errors in the proving API spec. A new variant fails to compile until those matches and the spec are updated.

strum is added as a dev-dependency; Cargo.lock reflects that.

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