feat: add focused empty and regex assertions (#136)

[adity982]

Closes #136

Summary

• add auto-retrying isFocused() and isEmpty() assertions
• add Pattern overloads for full text/value matching and partial text matching
• re-resolve stale or late elements on every poll and include the locator plus last observed value in regex/empty/focus failures
• document the assertion surface and add the changelog entry

Validation

• mvn -q -Dtest=LocatorAssertBehaviorTest,SeleniumAssertTest test (11 passed)
• mvn -q test (642 passed, 0 failed/error/skipped)
• mvn -q verify completed compilation and all tests, then stopped at the release-only GPG signing goal because GPG is not installed on this host
• git diff --check

No browser-backed sample was needed: the focused tests exercise WebDriver polling, focus ownership, form emptiness, regex semantics, stale-element re-resolution, and failure diagnostics directly.

[shossain786]

Thanks, Aditya, this is a clean piece of work. I checked out the branch and ran the full suite: 642 run, 0 failures, matching your numbers.

Review notes:

• isFocused() / isEmpty() / the Pattern overloads all re-resolve on every poll and treat stale elements as "try again", as the spike(locator): stale-proof click() — re-resolve on every poll #67/feat(locator): terminal actions wait for presence and re-resolve on every poll #68 contract requires.
• The failure message names the locator and the last observed value, which was the acceptance criterion, and the existing poll(condition, message) messages are unchanged.
• The tests cover live state, full vs partial match, stale re-resolution and the failure message. Javadoc, changelog and guide line are all there.

Two things before it can merge:

1. It's still marked as a draft. Mark it ready for review whenever you're happy with it.
2. feat(locator): waitFor, allTextContents, element screenshot (#135) #150 (Pushpak's Locator additions) touches the same lines of docs-site/docs/changelog.md and semantic-locators.md, so whichever lands second will need a quick rebase. Nothing to do yet; I'll tell you if it ends up being yours.

I found nothing blocking in the code itself. Thanks for picking this one up!