Add regression tests and contributor docs for contract patterns

[Akanimoh12]

Summary

This PR adds regression coverage for the sell slippage boundary, creator fee recipient authorization, and the creator TTL extension path after successful and failed sells. It also adds a contributor guide that documents the repo's expected contract function patterns and CI checks.

Changes

• Added regression coverage for exact-equality sell slippage handling.
• Added authorization tests for creator fee recipient rotation.
• Added coverage for creator TTL extension after successful sells and no extension after failed sells.
• Added a contributor guide for new contract functions and linked it from the main contributing guide.

Closes

Closes #500
Closes #501
Closes #502
Closes #503

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