fix(code-gen): replace Manager proxies with bounded IPC

[ananthsub]

Summary

• replace each verification's multiprocessing.Manager helper and two ListProxy objects with one versioned payload over a one-way pipe
• pass already validated unit-test dictionaries across Ray instead of dumping to JSON and parsing them again
• cap the input-scaled child backstop at a configurable 600 seconds and cap accepted result payloads at 16 MiB
• preserve one fresh isolated child per submission and request non-forceful Ray cancellation when the caller disconnects

Correctness and reliability

Timeout, child crash, EOF, malformed or oversized payload, invalid tests, and interruption fail closed with the existing failed-test shape. Every path closes both pipe endpoints and kills/joins a live child. The worker remains single-use because LiveCodeBench's reliability guard mutates process-global state.

Validation

• uv run --extra dev pytest resources_servers/code_gen/tests -q — 73 passed
• scoped pre-commit — passed
• real child tests verify call-based/plain-dict and legacy JSON input parity
• 16 forced child crashes leave no descendants or additional file descriptors

Focused IPC benchmark

For a ten-result payload over 50 local runs:

• Manager helper + ListProxy: 7.464 ms median, 8.758 ms p95, two child processes per verification
• one-way Pipe: 1.167 ms median, 1.636 ms p95, one child process per verification

This isolates process/IPC overhead; it is not an end-to-end rollout throughput claim.

Closes #3028