[LyViolz]

Fixes #696

Adds a checkbox to each breakpoint in the Breakpoints pane, so that a breakpoint can be turned off without removing it, like in Chrome DevTools. The checkbox calls the VM service setBreakpointState RPC. That RPC landed in dart-lang/sdk@2a2d83e, but DevTools never used it.

• BreakpointManager.setBreakpointEnabled calls setBreakpointState. The VM replies with a BreakpointUpdated event, and DevTools now uses that event to update breakpointsWithLocation.
• Disabled breakpoints are drawn in a muted color, both in the list and in the gutter.
• A disabled breakpoint stays disabled when breakpoints are re-established after a hot reload or a hot restart.
• The checkbox is hidden for Dart web apps, because DWDS throws UnimplementedError for setBreakpointState (pkg/dwds/lib/src/services/proxy_service.dart).
• The BreakpointResolved handler changed the list held by the ValueNotifier in place and then assigned the same list back, so listeners were not notified. It now builds a new list. BreakpointUpdated uses the same code path, and the new breakpoint_manager_test.dart fails without this change.

Screenshots: TODO before marking ready for review.

Pre-launch Checklist

General checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I updated/added relevant documentation (doc comments with ///).

Issues checklist

• I listed at least one issue that this PR fixes in the description above.
• I listed at least one issue which has the contributions-welcome or good-first-issue label.
• I did not list at least one issue with the contributions-welcome or good-first-issue label. I understand this means my PR might take longer to be reviewed.

Tests checklist

• I added new tests to check the change I am making...
• OR there is a reason for not adding tests, which I explained in the PR description.

AI-tooling checklist

• I did not use any AI tooling in creating this PR.
• OR I did use AI tooling, and...
  • I read the AI contributions guidelines and agree to follow them.
  • I reviewed all AI-generated code before opening this PR.
  • I understand and am able to discuss the code in this PR.
  • I have verifed the accuracy of any AI-generated text included in the PR description.
  • I commit to verifying the accuracy of any AI-generated code or text that I upload in response to review comments.

Feature-change checklist

• This PR does not change the DevTools UI or behavior and...
  • I added the release-notes-not-required label or left a comment requesting the label be added.
• OR this PR does change the DevTools UI or behavior and...
  • I added an entry to packages/devtools_app/release_notes/NEXT_RELEASE_NOTES.md.
  • I included before/after screenshots and/or a GIF demo of the new UI to my PR description.
  • I ran the DevTools app locally to manually verify my changes.