enterprise hook: generic 4xx still returns type/param as the string "None"; allow vLLM 'max_tokens must be at most N' through #121

Follow-ups from the nan-devops#636 reviews:

1. Enterprise type/param bug. The enterprise usage hook still returns the generic sanitised 4xx with "type":"None","param":"None", the literal string. Community fixed this in #636: param is JSON null and type is mapped by status. Port the same mapping to the enterprise hook, which is a separate program, and tighten its parity test afterwards.
2. 5xx type. In both tenants 5xx still carries "type":"None"; map it to server_error.
3. 429 type consistency. Our own 429s now say rate_limit_error, while upstream-origin ones keep throttling_error. Pick one.
4. Useful vLLM message is swallowed. The generic sanitiser replaces vLLM's max_tokens must be at most N (no vendor information in it) with the generic 400. Let that message through.

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