🛡️ Sentinel: [MEDIUM] Add input length limits to API requests

[n24q02m]

🚨 Severity: MEDIUM
💡 Vulnerability: Unbounded input fields in Pydantic request models (AskRequest, DistillRequest, GraphIngestRequest). Default Pydantic strings and lists allow indefinitely large payloads, posing a risk of memory exhaustion.
🎯 Impact: Malicious actors could send extremely large strings or nested lists, causing Denial of Service (DoS) by exhausting server memory or parsing resources.
🔧 Fix: Implemented length boundaries. Strings are capped at 1,000 characters using typing.Annotated[str, Field(max_length=1000)]. List sizes are bounded at 10,000 for tuples and 1,000 for simple string elements.
✅ Verification: Ran uv run pytest to ensure legitimate requests are not rejected. Passed formatting (uv run ruff format .) and lint checks (uv run ruff check .).

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PR created automatically by Jules for task 12817441361953420958 started by @n24q02m

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