Enable diagnostics widget when present in dashboard layout

[multiplex55]

Motivation

• Ensure diagnostics data and rendering remain available when a diagnostics widget is present in the dashboard layout even if the global show_dashboard_diagnostics flag is false.

Description

• Add has_diagnostics_widget helper and compute show_diagnostics_widget = self.show_dashboard_diagnostics || has_diagnostics_widget when constructing DashboardContext, and only collect a diagnostics snapshot when that combined flag is true.
• Keep the existing skip guard in Dashboard::ui (so diagnostics rendering can still be skipped when the widget is genuinely absent) and add a unit test that builds a minimal dashboard with a diagnostics slot to validate the computed flag.

Testing

• Added unit test diagnostics_widget_layout_enables_context_flag in src/gui/mod.rs which asserts the computed flag is enabled when a diagnostics slot is present, and no automated test suite was executed as part of this change.

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Codex Task