Fix flaky test on site theming mcp tool

[sf-cboscenco]

Summary

Fix flaky test on site theming mcp tool

Testing

This is a test fix.

Dependencies

• No net-new third-party dependencies were added
• If net-new third-party dependencies were added, rationale/discussion is included and 3pl-approved is set by a maintainer

---

• Tests pass (pnpm test)
• Code is formatted (pnpm run format)

[sf-cboscenco]

Root cause: flaky testDir and early return in initialize
The failures come from two things:

1. testDir can be identical across tests
   testDir is created with:
   theming-store.test.ts
   Lines 23-23
   testDir = path.join(tmpdir(), b2c-theming-store-test-${Date.now()});
   Date.now() only changes every millisecond. When tests run quickly, several can share the same testDir.
2. initialize skips when the root is unchanged
   In theming-store.ts:
   theming-store.ts
   Lines 364-366
   if (this.initializedForRoot === root) { return; }
   If two tests get the same testDir, the second call to initialize(testDir) returns immediately and never processes the new THEMING_FILES from the current test.
   How this breaks the failing tests
   "should resolve absolute paths in THEMING_FILES"
   Sets THEMING_FILES and calls initialize(testDir).
   If testDir matches the previous test, initialize returns early and never loads the custom file.
   has('absolute-theming') stays false.
   "should clear and re-load when root changes"
   Same pattern: if testDir collides with another test, initialize may not run as expected and second-root is never loaded.
   Evidence
   Running the suite 20 times showed intermittent failures (sometimes 1, sometimes 2), which matches this timing-based collision.
   Fix
   Make testDir unique per test so initialize always runs. For example:
   testDir = path.join(tmpdir(), b2c-theming-store-test-${Date.now()}-${Math.random().toString(36).slice(2, 9)});
   Or use a counter:
   let testCounter = 0;// in beforeEach:testDir = path.join(tmpdir(), b2c-theming-store-test-${Date.now()}-${++testCounter});
   Either approach prevents testDir collisions and removes the flakiness. I can switch to Agent mode and apply this change if you want.