Add Debug.Assert for missing OverlayDialogHost with setup guidance

[Copilot]

When developers call OverlayDialog.Show() or Drawer.Show() without properly setting up an OverlayDialogHost, the operations silently fail. This adds a debug-mode assertion to surface the issue immediately with actionable guidance.

Changes

• Added Debug.Assert in OverlayDialogManager.GetHost() when host lookup returns null
• Assert message provides two setup paths:
  1. Use UrsaWindow or UrsaView (includes OverlayDialogHost in templates)
  2. Manually add OverlayDialogHost with HostId property
• Links to documentation: https://docs.irihi.tech/ursa/docs/advanced/dialog-and-drawer/overlay-dialoghost

Example

// Without proper setup, this now asserts in debug builds:
OverlayDialog.Show<MyView, MyViewModel>(viewModel);

// Debug output:
// OverlayDialogHost not found. Please ensure the host is properly set up:
// 1. Use UrsaWindow or UrsaView which includes OverlayDialogHost in their control templates, or
// 2. Add an OverlayDialogHost control with a HostId property to your view.

Debug-only, no impact on release builds.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Assert overlay dialog host is found in debug mode</issue_title>
<issue_description>When retrieving OverlayDialogHost from host manager, if the host is not found, it's likely developer is not setting up the host correctly. We should call Debug.Assert, explain that host should be setup correctly with UrsaWindow/UrsaView or a self-setup OverlayDialogHost with property HostId, and redirect developer to documentation https://docs.irihi.tech/ursa/docs/advanced/dialog-and-drawer/overlay-dialoghost. </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Assert overlay dialog host is found in debug mode #862

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[github-actions]

Test Results

715 tests  ±0   715 ✅ ±0   15s ⏱️ -1s
  2 suites ±0     0 💤 ±0 
  2 files   ±0     0 ❌ ±0 

Results for commit 3621313. ± Comparison against base commit 975757d.

♻️ This comment has been updated with latest results.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Ursa	48%	34%	8150	✔
Ursa.Themes.Semi	61%	68%	1018	✔
Summary	53% (10682 / 20129)	37% (2638 / 7056)	9168	✔