[Docs] Add LeetCode API Payload Example to Proof Connector Spec

• Labels: good first issue, documentation, difficulty:beginner, area:core
• Target File: docs/INTEGRATIONS.md
• Estimated Time: 5–10 minutes
• Difficulty: Level 1 (Beginner)

Why This Matters

PACT verifies proof-of-work accountability through external platforms like LeetCode and GitHub. Providing a concrete mock JSON payload in docs/INTEGRATIONS.md helps contributors understand how verification data is parsed without needing live API tokens.

Current Behavior

docs/INTEGRATIONS.md describes the LeetCode GraphQL connector conceptually, but does not include a sample GraphQL JSON response for submission verification.

Requested Change

Add a fenced JSON code block in docs/INTEGRATIONS.md under the LeetCode section showing a sample GraphQL response payload containing recentSubmissionList with titleSlug, statusDisplay: "Accepted", and timestamp.

Acceptance Criteria

• Sample JSON payload block added to docs/INTEGRATIONS.md.
• JSON is valid and matches the fields handled in src/lib/integrations/leetcode.ts.

Verification

node scratch/check-links.mjs

Contributor Notes

Documentation-only task.

Start with the LeetCode section in docs/INTEGRATIONS.md and compare its described fields with src/lib/integrations/leetcode.ts. Add a valid fenced JSON example containing recentSubmissionList, titleSlug, statusDisplay set to "Accepted", and timestamp. Run node scratch/check-links.mjs and confirm the acceptance criteria are met.