feat: public read API on ClipboardBinding (#24480) (CP: 25.2)

[vaadin-bot]

This PR cherry-picks changes from the original PR #24480 to branch 25.2.

Original PR description

Clipboard.onClick(button).read(onPayload, onError) — and the
single-field convenience variants readText / readHtml — make the
read side reachable from the same binding entry point as the write
side, instead of forcing callers to instantiate the internal
ReadFromClipboardAction themselves.

ClipboardPayload moves out of trigger.internal into the public
clipboard package so it can sit in the read methods' signatures
without leaking an internal type, mirroring how ClipboardContent
already lives there for the write side. The private bind() helper
on ClipboardBinding is generalised to accept any Action so it can
route both write and read actions.

The read IT view is restructured into headed sections and gains
two new buttons (read-text, read-html); the IT picks up matching
scenarios that use the existing resolving-clipboard shim.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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

See analysis details on SonarQube Cloud

[github-actions]

Test Results

 1 437 files  ±0   1 437 suites  ±0   1h 21m 7s ⏱️ - 1m 45s
10 105 tests +5  10 037 ✅ +5  68 💤 ±0  0 ❌ ±0 
10 577 runs  +5  10 508 ✅ +5  69 💤 ±0  0 ❌ ±0 

Results for commit e463684. ± Comparison against base commit b513b5e.

[vaadin-bot]

This ticket/PR has been released with Vaadin 25.2.0-beta2.