starknet_api: avoid double-parse in TryFrom<ProofFacts> for SnosProofFacts error path

[gkaempfer]

Summary

Follow-up optimization to #14598.

PR #14598 introduced a custom Debug impl for ProofFacts that internally calls ProofFactsVariant::try_from. This exposed a latent inefficiency in TryFrom<ProofFacts> for SnosProofFacts: its error arm used {proof_facts:?} in the format string, which now triggers a redundant second call to ProofFactsVariant::try_from — the inner error from the first call was being discarded and the data re-parsed just to build the error message.

Change

• Replace the wildcard _ error arm with explicit matching: propagate the inner Err via ? (reusing its already-descriptive message), and handle the Empty variant separately with a clear message.
• Add three tests: success path, empty-input rejection, and inner-error propagation (ensuring the generic wrapper string is no longer produced).

Before / After

Before — error path triggered two parses:

match ProofFactsVariant::try_from(&proof_facts) {
    Ok(ProofFactsVariant::Snos(snos)) => Ok(snos),
    _ => Err(StarknetApiError::InvalidProofFacts(format!(
        "Invalid SNOS proof facts: {proof_facts:?}",  // ← second try_from here
    ))),
}

After — inner error propagated directly:

match ProofFactsVariant::try_from(&proof_facts)? {
    ProofFactsVariant::Snos(snos) => Ok(snos),
    ProofFactsVariant::Empty => Err(StarknetApiError::InvalidProofFacts(
        "expected SNOS proof facts, got empty".to_string(),
    )),
}

Expected impact

Eliminates one redundant ProofFactsVariant::try_from call on every error path through TryFrom<ProofFacts> for SnosProofFacts. Also produces more specific error messages (the inner parse error explains exactly what was wrong, rather than a generic wrapper).

Test plan

RUSTC_WRAPPER="" SEED=0 cargo test -p starknet_api --features testing -- fields_test

All 9 tests pass, including 3 new ones added by this PR.

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[reviewable-StarkWare]

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