docs: add minimum viable test structure guide for new contract functions

[wheval]

Summary

Adds contributor documentation that defines the minimum test categories expected for any new contract function and includes example structures for read-only and write entrypoints.

Purpose / Motivation

Contributors adding new contract functions had no single reference for the baseline tests reviewers expect. This doc sets a consistent quality bar by listing required categories — happy path, error cases, and state assertions — and pointing to the shared contract_test_env helpers.

Changes Made

• Add docs/minimum-viable-test-structure.md with:
  • A table of minimum test categories and when each applies
  • Example test structures for a read-only function (get_key_symbol) and a write function (set_protocol_fee_recipient)
  • A pre-PR checklist and links to related error/read-only docs
• Link the new guide from CONTRIBUTING.md (integration test helpers section and documentation list)
• Cross-reference the guide from creator-keys/tests/contract_test_env/mod.rs

Docs-only change; no contract or test code modified.

How to Test

1. Open docs/minimum-viable-test-structure.md and confirm the three minimum categories (happy path, error cases, state assertions) are clearly listed.
2. Verify both example sections (read-only and write) include setup, execution, and assertions aligned with existing tests in creator-keys/tests/.
3. From CONTRIBUTING.md, follow the link to the new doc and confirm it resolves.
4. Run cargo test --workspace — should pass unchanged (no code changes).

Breaking Changes

None.

Related Issues

Closes #305

Checklist

• Code builds successfully
• Tests added/updated (N/A — docs-only)
• No console errors
• Documentation updated (if needed)

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