perf(api): use crypto.randomUUID for mock response IDs

[shenald-dev]

Summary

This pull request switches the mock response ID generation from a pseudo-random approach using Math.random to Node.js's native crypto.randomUUID().

Why it matters

The previous ID generation method (Math.random().toString(36).substr(2, 9)) generated short, non-cryptographic pseudo-random strings. For an API gateway under high load, this introduces a mathematically significant risk of ID collisions. Switching to UUID v4 ensures cryptographic uniqueness while natively improving string generation performance in Node.js.

Verification

• Validated speed improvements using Node.js REPL (performance.now()).
• Updated tests/api.test.js to ensure the new length of response IDs is handled correctly.
• Ran npm test and npm run benchmark successfully.

Remaining Risk

None identified. The length of the ID increased, but the OpenAI specification does not strictly define maximum ID lengths for chatcmpl- string identifiers.

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PR created automatically by Jules for task 2786242430257256644 started by @shenald-dev

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