[DeepDev69x3]

Summary

Adds comprehensive deterministic unit tests covering pagination cursor tampering defenses, 64-bit integer overflows, boundary sequences, and mixed sort direction query parsing:

• Cursor tampering rejection: asserts HTTP 400 and ErrorCode.VALIDATION_ERROR for non-base64 input, non-JSON decoded payloads, missing or non-string createdAt, invalid ISO date strings, missing or non-string seq, and unparseable BigInt sequence values.
• 64-bit integer overflow support: asserts round-trip encoding and decoding for max signed 64-bit integer (9223372036854775807n), max unsigned 64-bit integer (18446744073709551615n), and zero sequence (0n).
• Keyset WHERE clause generation with boundary sequences (0n, 18446744073709551615n) and Unix epoch timestamps (1970-01-01T00:00:00.000Z).
• Mixed sort direction parsing: handles standard directions (asc, desc), compound column-direction specifiers (created_at:desc, amount:asc), and whitespace/empty input fallback in parseQueryParams and parseCursorQueryParams.

Test Coverage

• npm test -- src/utils/__tests__/pagination.test.ts: 29/29 tests passing 100% green.
• npm run build: 0 errors.
• npm run lint: 0 errors.

Closes #2086

[github-actions]

Malware detection scan flagged this pull request

The scan found 2 indicator(s) in the PR diff. This PR is blocked until a maintainer reviews the flagged files.

• backend/src/utils/__tests__/pagination.test.ts — encoded-payload-execution (high)
• backend/src/utils/__tests__/pagination.test.ts — hidden-appended-config-payload (high)

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