Gate diagnostics snapshot on flag OR widget presence and add tests

[multiplex55]

Motivation

• Avoid unnecessary Dashboard.diagnostics_snapshot() calls while still providing diagnostics to widgets when present by gating snapshotting on either the dev flag or a diagnostics widget being configured.
• Make behavior explicit and testable so the dashboard context includes diagnostics only when required.

Description

• Change the diagnostics assignment in src/gui/mod.rs to compute diagnostics as Some(self.dashboard.diagnostics_snapshot()) when self.show_dashboard_diagnostics || has_diagnostics_widget is true, and None otherwise.
• Add two unit tests to the existing tests in src/gui/mod.rs: diagnostics_context_includes_snapshot_when_widget_present and diagnostics_context_omits_snapshot_when_disabled_and_missing_widget to cover both inclusion and exclusion cases.
• The change touches src/gui/mod.rs only and reuses the existing has_diagnostics_widget() helper and Dashboard.diagnostics_snapshot() accessor.

Testing

• No automated test suite was executed as part of this change; the two new unit tests were added but not run here: diagnostics_context_includes_snapshot_when_widget_present and diagnostics_context_omits_snapshot_when_disabled_and_missing_widget in src/gui/mod.rs.
• Existing tests were not executed in this rollout.

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Codex Task