Skip missing session assemblies when navigating on launch

[siegfriedpammer]

Problem

Launching ILSpy with a navigation target (--navigateto/-n, or the VS add-in IPC path) crashes at startup with an unhandled DirectoryNotFoundException when the restored session still references an assembly whose file has since been deleted or moved:

System.IO.DirectoryNotFoundException: Could not find a part of the path '.../test.dll'.
   at ICSharpCode.ILSpyX.LoadedAssembly.GetMetadataFileAsync() ...
   at ICSharpCode.ILSpy.AssemblyTree.AssemblyTreeModel.NavigateOnLaunchAsync(...)
   at ICSharpCode.ILSpy.AssemblyTree.AssemblyTreeModel.HandleCommandLineArgumentsAsync(...)

NavigateOnLaunchAsync eagerly pre-loads every relevant assembly's metadata before resolving the target. When no assembly is passed positionally, "relevant" is the whole restored session list, so a single gone file aborts the entire launch. (Passing an assembly on the command line masks it, because then only that one assembly is relevant.)

Fix

Use the non-throwing GetMetadataFileOrNullAsync() in the pre-load loop (both the N: namespace branch and the type/member branch). A missing or unreadable assembly now resolves to null and is skipped; the subsequent entity search already tolerates null (it uses GetMetadataFileOrNull()), so a still-present navigation target resolves as before.

Test

Adds a headless regression test that injects a session assembly whose file does not exist, then navigates on launch and asserts the launch completes without throwing and the present target (System.Linq.Enumerable) is still selected. The test fails (reproducing the DirectoryNotFoundException) before the fix and passes after. Full ILSpy.Tests suite green.

🤖 Generated with Claude Code