feat: simulate clipboard paste and file paste in browserless tests

[mcollovati]

Extend ClipboardSimulator to drive clipboard read paths.
pasteInto fires an onPaste DOM event with simulated text/html, and
pasteFilesInto simulates onFilePaste by invoking the resolved
UploadHandler for each PastedFile.

Extract the upload plumbing shared with UploadTester into an internal
UploadSimulation helper (no behavioral change to UploadTester).

[mcollovati]

Part of a 4-PR split (2 PRs per repo).

• Stacked on feat: simulate Flow triggers and actions for clipboard testing #146 — its base branch is pr/trigger-simulation-core, so the diff shown is incremental; please review/merge after feat: simulate Flow triggers and actions for clipboard testing #146.
• Depends on chore: expose clipboard hooks for test simulation flow#25046 (clipboard hooks) in addition to the seam from feat: simulate Flow triggers and actions for clipboard testing #146. CI here stays red until both flow PRs merge and a new 25.3-SNAPSHOT is published; verified locally against a local flow-server install of A+B.
• Verified locally: ClipboardSimulationTest 10/10, UploadTesterTest 12/12, UploadTesterDeprecatedAPITest 9/9 (UploadSimulation extraction is a no-behavior-change refactor).