#14249: slack: add DM policy matrix regressions (open/allowlist/pairing/disabled)

## Problem Fixes #13294. Slack DM policy behavior needs explicit regression coverage to prevent fail-open routing and incorrect pairing behavior across policy modes. ## What Changed - Added a DM policy matrix helper in Slack monitor tests. - Added policy-path tests for `open`, `allowlist`, `pairing`, and `disabled`. - Asserts pairing is only triggered for `pairing` mode and blocked paths do not process replies. ## Validation - `pnpm vitest run src/slack/monitor.tool-result.forces-thread-replies-replytoid-is-set.test.ts` - Passed (8 tests) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds regression test coverage for Slack DM policy matrix behavior across all four modes (`open`, `allowlist`, `pairing`, `disabled`). A shared `runDmMessageWithPolicy` helper consolidates DM event setup with an empty `allowFrom` list, and four new test cases verify that each policy mode produces the correct routing outcome — ensuring `open` processes messages regardless of allowlist, `allowlist` and `disabled` block without triggering pairing, and only `pairing` mode invokes the pairing flow. - Added `runDmMessageWithPolicy` helper that configures DM policy and fires a simulated DM event - Added 4 regression tests covering each `DmPolicy` variant with correct assertion boundaries - All test assertions align with the existing enforcement logic in `prepare.ts` <h3>Confidence Score: 5/5</h3> - This PR is safe to merge — it only adds test code with no production changes. - The PR adds only regression tests in a single test file. No production code is modified. The test assertions correctly match the existing DM policy enforcement logic in prepare.ts. The helper function is well-structured and the four policy paths are all covered with appropriate expectations. - No files require special attention. <!-- greptile_other_comments_section --> <!-- /greptile_comment -->