V2 team Suspicious Sender Watchlist - Testing and documentation

[iyanumajekodunmi756]

Summary

Fixes #673 — Adds contributor-friendly testing and documentation for the Suspicious Sender Watchlist tool.

Deliverables

Documentation

• CONSTRAINTS.md: Contributor constraints defining what can/cannot be modified
• docs/test-plan.md: Automated test plan, manual review checklist, edge cases covered
• docs/review-notes.md: Review notes for OSS contributors with known limitations and follow-up work
• tests/README.md: Test structure, running instructions, fixtures table, best practices

Expanded Test Coverage

• Edge cases: empty state, null/undefined filter fields, combined filters (riskLevel + status + search)
• Config options: delayMs (async operations), failureRate (error handling)
• Mutation sequences: sequential add → update → dismiss → remove → metrics
• Idempotency: dismissing already-dismissed entries succeeds
• Field preservation: updateRisk preserves all other fields
• Descriptive errors: all operations reject with meaningful messages for unknown ids

Validation

node --test tools/v2/team/suspicious-sender-watchlist/tests/watchlist.test.mjs

All 43 tests pass (was 29 before this contribution).

Scope

All changes are confined to tools/v2/team/suspicious-sender-watchlist/. No main app or other tool files modified.

Review Notes

See docs/review-notes.md for detailed reviewer guidance.

Closes #673