Raise test coverage to ~55%

[bbatsov]

#4 on the list. Covers the command/dispatch layer - the part a regression from all the recent churn would hit hardest:

• helm-projectile entry point (in-project sources vs. switch-project fallback)
• helm-projectile-grep argument hand-off to the deferred runner
• helm-projectile-file-persistent-action file preview
• the single-candidate paths of helm-projectile--find-file-dwim-1 and helm-projectile--find-other-file-1
• helm-projectile-dired-files-delete-action, the remaining virtual-Dired action, against a sandbox project

Also makes the sandbox fixture resolve its root to a truename so it matches the paths Dired reports back (on macOS the temp dir lives under a symlinked /var -> /private/var).

Coverage: ~46% -> ~55% (75 specs, up from 68). Remaining gaps are mostly dired-files-add-action and the rg/ag switch-project actions - a possible future pass.

• The commits are consistent with our contribution guidelines
• The new code is not generating bytecode or M-x checkdoc warnings
• You've updated the changelog
• You've updated the readme (no user-facing change)

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.17%. Comparing base (68d357c) to head (846065d).
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
+ Coverage   46.60%   55.17%   +8.57%     
==========================================
  Files           1        1              
  Lines         560      560              
==========================================
+ Hits          261      309      +48     
+ Misses        299      251      -48     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.