fix(AppLifecycle): coalesce multi-file activation redirections into single event

[MuyuanMS]

Summary

Fixes #5066 — App activation fires multiple times when launching multiple files

When a user selects N files and opens them with a WinAppSDK single-instanced app, N processes launch. N-1 processes redirect to the main instance via RedirectActivationToAsync(). Previously, the main instance fired N separate Activated events — each containing 1 file — instead of 1 event containing all N files.

Root Cause

ProcessRedirectionRequests() in AppInstance.cpp had a simple while-loop that dequeued each redirection request and fired m_activatedEvent independently for each one.

Fix

dev/AppLifecycle/FileActivatedEventArgs.h

• Added a new constructor FileActivatedEventArgs(verb, IVector<IStorageItem>) for coalesced multi-file activations
• Updated comments to clarify single-file vs. multi-file paths

dev/AppLifecycle/AppInstance.cpp

Rewrote ProcessRedirectionRequests() to:

1. Categorize incoming requests as file activations vs. non-file activations
2. Collect all file activations into a shared coalescedFiles vector
3. Wait 150ms coalescing window for late-arriving redirections (other processes may not have enqueued yet)
4. Merge all collected files into a single FileActivatedEventArgs
5. Fire once — one Activated event with all files
6. Signal cleanup events for all coalesced redirecting processes so they can exit

Non-file activations continue to fire immediately, preserving backward compatibility.

Testing

• WindowsAppRuntime_DLL.vcxproj builds cleanly (x64 Release)
• Manual verification: opening multiple files should now produce a single Activated event with all files in the Files collection

Notes

• The 150ms coalescing window adds a small delay to single-file activations but ensures multi-file opens are properly coalesced
• The branch also contains .github/ infrastructure files unrelated to this fix

[benstevens48]

Please don't add a forced delay for file activation. The issue can be worked around (at least in packaged apps) by using the MultiSelectModel property of uap3:FileTypeAssociation in the appxmanifest and setting it to Player as follows

<Extensions>
  <uap3:Extension Category="windows.fileTypeAssociation">
    <uap3:FileTypeAssociation Name="my-extensions" MultiSelectModel="Player">
      ..

This means the app is only launched once (with multiple files provided to the file activation args).

If you are going to add a delay, please make this optional as start-up speed is very important to me.