fix(#810): validate ratePerSecond/depositedAmount before BigInt coercion

[BernardOnuh]

Description

Fixes a validation bug in createStream: BigInt(ratePerSecond) and BigInt(depositedAmount) were called before any presence/format validation. A non-numeric value throws SyntaxError and a missing value throws TypeError, but the surrounding catch block only mapped RangeError to 400 — every other conversion failure fell through to the generic 500 handler.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• 🧪 Test addition or update

Related Issues

Closes #810

Changes Made

• Added parseRequiredBigIntField() in stream.controller.ts: validates a field is present (not undefined/null/'') before attempting BigInt() coercion, and wraps the coercion itself in try/catch so any conversion failure (SyntaxError, TypeError, etc.) is normalized into a StreamValidationError.
• createStream now validates ratePerSecond and depositedAmount presence/format before the <= 0n checks, and maps any StreamValidationError to a 400 response with a descriptive message instead of letting it fall through to 500.
• Added 4 new tests to stream.controller.test.ts:
  • non-numeric ratePerSecond → 400 with a validation error (not 500)
  • non-numeric depositedAmount → 400 with a validation error (not 500)
  • missing ratePerSecond → 400, not 500
  • missing depositedAmount → 400, not 500

Testing

Test Coverage

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed

Test Steps

1. cd backend && npx prisma generate && npx vitest run tests/stream.controller.test.ts
2. All 13 tests in stream.controller.test.ts pass (7 in the createStream block)
3. Verified regression coverage by temporarily reverting the source fix and confirming all 4 new tests fail with 500 against the old (buggy) code, then re-confirming they pass with the fix applied

Breaking Changes

None. Response bodies for already-valid requests are unchanged; only previously-500 responses for malformed/missing numeric fields now correctly return 400.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published (N/A)

Additional Notes

Scope limited to backend/src/controllers/stream.controller.ts per the issue's "Files to touch." Stellar address format validation (sender/recipient/tokenAddress) is explicitly out of scope per the issue and tracked separately.