New extension methods fails to provide the inline documentation and navigation (goto definition) fails (package references).

Please find below the case "IRL" (with all its complexity as there may be something in this exact context that is the issue...).

The declaring/consumed package is "CKli.Testing" (https://github.com/CK-Build/CKli/tree/57fb8993a696c7eb101adaed3f52676d8c3eb9aa/CKli.Testing) (Commit: 57fb8993, reverted back to the "legacy" mode after this commit).

The consuming package is "Plugins.Tests" (https://github.com/signature-opensource/CK-Stack/tree/ext-method-bug/CK-Plugins/Tests/Plugins.Tests).

## The legacy extension (works):
- Declaration: https://github.com/CK-Build/CKli/blob/57fb8993a696c7eb101adaed3f52676d8c3eb9aa/CKli.Testing/CKliTestHelperExtensions.cs#L312

- Usage: https://github.com/signature-opensource/CK-Stack/blob/ext-method-bug/CK-Plugins/Tests/Plugins.Tests/InitializationTests.cs#L69

F12 (Goto Definition) works as expected:

## The modern one (failing):
- Declaration: https://github.com/CK-Build/CKli/blob/57fb8993a696c7eb101adaed3f52676d8c3eb9aa/CKli.Testing/CKliTestHelperExtensions.cs#L245

- Usage: https://github.com/signature-opensource/CK-Stack/blob/ext-method-bug/CK-Plugins/Tests/Plugins.Tests/InitializationTests.cs#L45

F12 (Goto Definition) goes to the top of the declaring file:

_Originally posted by @olivier-spinelli in [#79006](https://github.com/dotnet/roslyn/issues/79006#issuecomment-3839415256)_