fix: input validation DoS and L1 cache correctness

[shenald-dev]

• Security: Added length validation to isValidModel before calling .trim() to mitigate a potential event-loop-blocking Denial of Service (DoS) vulnerability via excessively large strings.
• Reliability/Correctness: Fixed an L1 cache initialization bug in heavyComputation. lastIterations and lastResult are now initialized to undefined instead of null to ensure correct cache misses when null is intentionally passed as an argument.
• Verification: Added comprehensive tests to tests/heavy_computation.test.js and tests/api_robustness.test.js to prevent regressions for both edge cases.

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PR created automatically by Jules for task 16850898484221337046 started by @shenald-dev

[google-labs-jules]

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[shenald-dev]

Looks good, merging this in. CI is green and everything checks out 👍

Adds input length validation to prevent DoS and fixes L1 cache initialization for correct null handling.

[google-labs-jules]

Looks good, merging this in. CI is green and everything checks out 👍

Adds input length validation to prevent DoS and fixes L1 cache initialization for correct null handling.

Acknowledged. Stopping work as requested.