fix(golden-layout): DH-22431: preserve focus indicator on nested dashboard tab

[mofojed]

• When a dashboard is nested inside another dashboard, focusin events bubble from the inner content up to the outer container's contentElement. The outer Tab's focusin handler would clear the inner tab's lm_focusin class and claim focus for itself, leaving the active tab indicator in the wrong place.
• _onTabContentFocusIn now ignores events whose target lives inside a nested LayoutManager. _onTabClick treats focus inside a nested layout as 'not in this container' so clicking the outer tab properly transfers the indicator back out.
• Tested with the following deeply nested layout:

from deephaven import ui

@ui.component
def deeply_nested_dashboard_component():
    """A dashboard nested inside a panel, which is inside another dashboard."""
    return ui.panel(
        ui.dashboard(
            ui.row(
                ui.panel(ui.text("Content Level 1"), title="Level 1"),
                ui.panel(
                    ui.dashboard(
                        ui.row(
                            ui.panel(ui.text("Content Level 2"), title="Level 2"),
                            ui.panel(ui.text("Deepest Content"), title="Deepest Panel"),
                        )
                    ),
                    title="Nested Dashboard Container",
                ),
            )
        ),
        title="Outer Dashboard",
    )
ui_deeply_nested_dashboard = deeply_nested_dashboard_component()

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.79%. Comparing base (49781a7) to head (d376de1).

Files with missing lines	Patch %	Lines
packages/golden-layout/src/controls/Tab.ts	90.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2664      +/-   ##
==========================================
+ Coverage   49.75%   49.79%   +0.04%     
==========================================
  Files         774      774              
  Lines       43881    43898      +17     
  Branches    11113    11308     +195     
==========================================
+ Hits        21832    21860      +28     
+ Misses      22031    21992      -39     
- Partials       18       46      +28     

Flag	Coverage Δ
unit	49.79% <90.00%> (+0.04%)	⬆️

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[dsmmcken]

Tested with example provided, confirmed working.