chore: expose clipboard hooks for test simulation

[mcollovati]

Widen visibility of the Clipboard paste event-data keys
(PASTE_TEXT_EXPR, PASTE_HTML_EXPR) and the file-paste hooks
(PASTE_UPLOAD_ATTRIBUTE_PREFIX, FILE_PASTE_FINISHED_EVENT) from
private to package-private so test frameworks can simulate paste and
file paste. No behavioral change.

[mcollovati]

Part of a 4-PR split of the trigger/action + clipboard test-support work (2 PRs per repo).

• Visibility-only change (private → package-private), no behavior change; there is no flow-side test because the behavior is exercised downstream in feat: simulate clipboard paste and file paste in browserless tests browserless-test#147 (paste + file-paste simulation).
• Independent of feat: add test-support seam for simulating triggers and actions #25045 — they touch different files and can merge in either order.

[github-actions]

Test Results

 1 423 files   - 18   1 423 suites   - 18   1h 34m 32s ⏱️ - 3m 35s
10 247 tests  - 59  10 180 ✅  - 59  67 💤 ±0  0 ❌ ±0 
10 694 runs   - 66  10 626 ✅  - 66  68 💤 ±0  0 ❌ ±0 

Results for commit bb9ba25. ± Comparison against base commit 626a502.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud