OptimizedNodeBridge should fail fast on spawn/script errors

[bbopen]

Summary

OptimizedNodeBridge does not validate the bridge script path or detect spawn errors. A missing/invalid scriptPath or pythonPath yields obscure errors later instead of a clear startup failure.

Failure mode

• scriptPath is missing or pythonPath is invalid.
• spawn emits error asynchronously; the worker is still added to the pool.
• Subsequent calls fail with unclear errors.

Evidence

• src/runtime/optimized-node.ts spawnProcess() calls spawn(...) without checking existsSync(scriptPath) or waiting for spawn/error signals.

Proposed fix

• Validate scriptPath exists before spawning (like NodeBridge).
• Await a spawn/ready signal and fail fast on error before adding the worker to the pool.
• Reject executeRequest with a clear BridgeProtocolError when startup fails.

Acceptance criteria

• Invalid pythonPath or missing scriptPath fails immediately with a clear error.
• No workers are added to the pool on spawn failure.
• Add a test that uses a missing script path and asserts a predictable error.

[bbopen]

✅ Resolved in PR #136: refactor: unify NodeBridge and OptimizedNodeBridge (ADR-0001)

The unified NodeBridge now validates scriptPath exists before spawning workers:

if (!existsSync(this.options.scriptPath)) {
  throw new BridgeProtocolError(`Python bridge script not found at ${this.options.scriptPath}`);
}

This check happens in doInit() before any workers are spawned, providing a clear error message for missing/invalid script paths.