Sanitize token usage, charge remaining credits, and add DB/accounting error handling in generateAnswer

[Abdulmuiz44]

Motivation

• Prevent crashes or integer overflows from malformed token usage values and avoid failing requests when users have partial credits left.
• Ensure we still save generated responses even if the user doesn't have enough credits to cover the full token cost.
• Surface friendly error messages to the client instead of throwing on DB failures and improve server-side logging for debugging.

Description

• Sanitize and clamp generationResult.usage.totalTokens into a finite integer and compute tokenCost with a safe 32-bit upper bound.
• Compute creditsToCharge as min(tokenCost, creditsRemaining) and remove the early rejection that returned an error when tokenCost exceeded remaining credits.
• Use creditsToCharge when calling incrementUserCredits and retain the retry loop, logging a usageAccountingWarning if accounting ultimately fails.
• Replace thrown errors on DB update/insert failures with console.error logging and a friendly error response so generated content is not lost.

Testing

• Ran the project's unit test suite with pnpm test and all tests passed.
• Ran a TypeScript type check/build with pnpm build and the build completed successfully.

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Codex Task