Fixes BreakpointLabelAction being enabled when selecting a BreakpointContainer

[r-mennig]

Currently, the BreakpointLabelAction is enabled when selecting a single BreakpointContainer (e.g. by grouping the breakpoints view by enablement state and selecting the "Enabled" group entry).

For such selections, the action has no effect because org.eclipse.debug.internal.ui.actions.breakpoints.BreakpointLabelAction.run(IAction) returns instantly if something other than a single org.eclipse.debug.core.model.Breakpoint is selected (which is the correct behavior here, since breakpoint containers are not renamable in general).

This PR disables the action if something other than a single breakpoint is currently selected.

Note: Making the action invisible would be more correct here, but this would require a restructure of the breakpoint view context menu since the action extension point does not allow making actions invisible.

How to test / reproduce

1. Set (at least) 1 active breakpoint and open the breakpoint view
2. Group breakpoints by enablement
3. Select the "Enabled" entry -> Context menu shows "Label" action, clicking the action has no effect

[github-actions]

Test Results

    54 files  ±0      54 suites  ±0   34m 8s ⏱️ - 2m 18s
 4 850 tests ±0   4 828 ✅ ±0   22 💤 ±0  0 ❌ ±0 
12 438 runs  ±0  12 282 ✅ ±0  156 💤 ±0  0 ❌ ±0 

Results for commit 8c24ecf. ± Comparison against base commit 7c138cf.

This pull request removes 387 and adds 276 tests. Note that renamed tests count towards both.

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[SougandhS]

Thanks for taking care of this 👍