Add toString() to Browser showing the underlying browser type

[vogella]

Summary

• Override Browser.toString() to append the active browser backend (e.g. webkit, edge, ie, safari) returned by getBrowserType().
• Makes it easy to identify which native browser implementation a Browser instance is using, both in the debugger and in log output, without having to drill into the internal webBrowser field.
• The implementation calls webBrowser.getBrowserType() directly (with a null guard) instead of the public getBrowserType() method, because the public method calls checkWidget() which toString() is not expected to trigger (e.g. when called on a disposed widget or from a non-UI thread, mirroring Widget.toString() behavior).

Test plan

• Open a Browser and inspect/print it; the result should look like Browser {} [webkit] (or the corresponding backend on Windows/macOS).
• Inspect a disposed Browser and confirm toString() does not throw.

🤖 Generated with Claude Code

[github-actions]

Test Results

  181 files   -  1    181 suites   - 1   27m 26s ⏱️ -18s
4 666 tests  - 57  4 625 ✅  - 75   20 💤  - 3  6 ❌ +6  15 🔥 +15 
6 755 runs   - 57  6 572 ✅  - 75  162 💤  - 3  6 ❌ +6  15 🔥 +15 

For more details on these failures and errors, see this check.

Results for commit 2ec0b30. ± Comparison against base commit f03d65d.

This pull request removes 57 tests.

org.eclipse.swt.graphics.ImageWin32Tests ‑ testDisposeDrawnImageBeforeRequestingTargetForOtherZoom
org.eclipse.swt.graphics.ImageWin32Tests ‑ testDrawImageAtDifferentZooms(boolean)[1] true
org.eclipse.swt.graphics.ImageWin32Tests ‑ testDrawImageAtDifferentZooms(boolean)[2] false
org.eclipse.swt.graphics.ImageWin32Tests ‑ testImageDataForDifferentFractionalZoomsShouldBeDifferent
org.eclipse.swt.graphics.ImageWin32Tests ‑ testImageShouldHaveDimesionAsPerZoomLevel
org.eclipse.swt.graphics.ImageWin32Tests ‑ testRetrieveImageDataAtDifferentZooms(boolean)[1] true
org.eclipse.swt.graphics.ImageWin32Tests ‑ testRetrieveImageDataAtDifferentZooms(boolean)[2] false
org.eclipse.swt.graphics.ImageWin32Tests ‑ test_getImageData_fromCopiedImage
org.eclipse.swt.graphics.ImageWin32Tests ‑ test_getImageData_fromImageForImageDataFromImage
org.eclipse.swt.tests.win32.Test_org_eclipse_swt_dnd_DND ‑ testByteArrayTransfer
…