Add a user-facing Cancel control for the scenario Open-from-folder scan

[jschick04]

Closes #656.

Problem

The scenario "Open from folder" scan (folder enumeration plus a parallel per-file channel probe) was only cancellable by disposing the dashboard. There was no busy/progress indicator or Cancel control, and for large or network folders the scan can take a noticeable amount of time.

Approach (persistent masthead Cancel chip)

A status chip in the dashboard masthead surfaces the scan and a Cancel control, driven by a dashboard field so it stays reachable regardless of which scenario or category tab is selected. It has three states:

• Scanning <scenario> folder for logs... + Cancel
• Cancelling folder scan for <scenario>... (after Cancel, while the in-flight probe drains)
• Opening logs for <scenario>... (past the commit boundary; no Cancel, because the open is not cancellable)

Browsing the scenario list and switching category tabs stay enabled; only the launch actions are busy-disabled, so there is no double-launch risk.

Changes

Service (ScenarioLaunchService)

• LaunchFromFolderAsync gains an optional Func<ScenarioFolderPhase, Task>? onPhase (defaulted null, so existing callers are unaffected).
• Reports Scanning once the picker returns a folder (cancellation is only meaningful from that point) and Opening once a nonempty match commits to opening logs, gated so Opening is never reported for an outcome that opens nothing.
• A cancellation recheck after the callback (independent of whether a callback is supplied) so a cancel that lands during the commit still wins before any filter dispatch or log open.

Dashboard (EmptyStateDashboard)

• Per-launch CancellationTokenSource linked to the component lifetime.
• Three-state chip plus focus management that keeps Cancel reachable and never strands keyboard focus on the document body (focuses Cancel on scan start; restores to the active category tab or the dashboard root otherwise).
• Guarded rendering so the busy state is reflected on the banner-retry path, which runs outside the dashboard event loop.
• The reactive-launch banner "Open from folder" retry is routed through the guarded wrapper so it no longer bypasses the busy guard.

Testing

• Runtime.Tests: 5 new phase-emission tests (Scanning/Opening boundaries, mid-scan cancel, no-match, cancel-during-Opening). Full suite: 2030/2030.
• UI.Tests (bUnit): 5 new tests covering chip visibility, decoupling from selection (reselecting a scenario keeps the chip), cancellation acknowledgement with no result dialog, the Opening relabel, and a banner-retry busy-state regression guard. Full suite: 1186/1186.
• Full-solution build: 0 warnings / 0 errors.

[Copilot]

Pull request overview

Adds a user-visible, masthead-level progress/cancel affordance for the “Open from folder” scenario scan, enabling in-place cancellation without disposing the dashboard. This is implemented by plumbing scan phase notifications from the runtime launch service into the dashboard so UI state (chip text, Cancel availability, focus restoration, and busy-guards) remains consistent even when the scan is initiated from non-standard paths (e.g., banner retry).

Changes:

• Extend ScenarioLaunchService.LaunchFromFolderAsync with an optional onPhase callback and introduce ScenarioFolderPhase to report Scanning/Opening boundaries (with cancellation re-check before committing).
• Add a persistent masthead “Scanning/Cancelling/Opening” chip with a Cancel control, per-launch linked CancellationTokenSource, and explicit focus management in EmptyStateDashboard.
• Add/adjust unit tests (runtime + bUnit UI) to cover phase emission boundaries, cancellation behavior, chip visibility/labeling, and banner-retry busy-guarding.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/Unit/EventLogExpert.UI.Tests/Dashboard/EmptyStateDashboardTests.cs	Adds bUnit coverage for the masthead scan chip states, Cancel behavior, focus/busy guarding, and banner-retry path.
tests/Unit/EventLogExpert.Runtime.Tests/Scenarios/ScenarioLaunchServiceTests.cs	Adds runtime coverage for phase reporting boundaries and cancellation timing (incl. cancel during Opening).
src/EventLogExpert.UI/Dashboard/EmptyStateDashboard.razor.css	Styles the new scan chip accent to distinguish it from the existing filter-applied chip.
src/EventLogExpert.UI/Dashboard/EmptyStateDashboard.razor.cs	Implements per-launch cancellation, guarded rendering for non-event-loop triggers, and focus restoration logic.
src/EventLogExpert.UI/Dashboard/EmptyStateDashboard.razor	Renders the masthead chip and Cancel button during the scanning window; makes the dashboard root focusable for safe focus fallback.
src/EventLogExpert.Runtime/Scenarios/ScenarioLaunchService.cs	Adds onPhase callback support, emits Scanning after picker success, emits Opening only when committing to open, and rechecks cancellation after callback.
src/EventLogExpert.Runtime/Scenarios/ScenarioFolderPhase.cs	Introduces the phase enum used by callers to drive user-visible scan state.
src/EventLogExpert.Runtime/Scenarios/IScenarioLaunchService.cs	Updates the service contract and documentation to include the optional onPhase callback.

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[ryanriesMSFT]

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