Add E2E coverage for the WinGetSourceCreator / IndexCreationTool layer

Type: Bug (missing test coverage)
Area: WinGetSourceCreator, IndexCreationTool

There is good coverage for the low-level index API, but not for the layer above
it that actually assembles a source.

WinGetUtilSQLiteIndex.cs (src/AppInstallerCLIE2ETests/WinGetUtil/) already
tests the WinGetUtil SQLite index directly - add/update/remove manifest,
open/close, PrepareForPackaging + consistency check. That coverage is solid,
but it feeds a single pre-merged manifest file straight into the index API.

What isn't covered is the WinGetSourceCreator / IndexCreationTool layer that
sits on top of it - the part that discovers manifests, copies them into the
working directory, handles multi-file manifests, and drives the index build.
Today that layer is only run as unverified test setup:

• TestIndex.GenerateE2ESource() (src/AppInstallerCLIE2ETests/Helpers/TestIndex.cs)
  calls WinGetLocalSource.CreateLocalSource(...).
• It runs from [OneTimeSetUp] in src/AppInstallerCLIE2ETests/SetUpFixture.cs
  to build the source that other tests search against, so it isn't itself a test
  of the source-creation path and the produced index isn't verified.
• The manifests it uses are all singleton, so the multi-file layout never goes
  through this path.

Because nothing exercises this layer directly, regressions in it can go
unnoticed for a long time. Two examples:

• #4181 (re-filed as #6426) - the tool looks for data in the WorkingDirectory
  that isn't there; open for ~2 years.
• #6433 - multi-file manifests can't be indexed.

Request

Could we add an E2E test for the WinGetSourceCreator / IndexCreationTool
path that builds a source from both a singleton and a multi-file manifest and
asserts the expected packages end up in the resulting index? This complements
the existing WinGetUtilSQLiteIndex tests rather than duplicating them.

We'd be glad to open a PR with this test if a maintainer is willing to review it.

Start with WinGetUtilSQLiteIndex.cs in src/AppInstallerCLIE2ETests/WinGetUtil/ and follow TestIndex.GenerateE2ESource() in src/AppInstallerCLIE2ETests/Helpers/TestIndex.cs, including its OneTimeSetUp use in SetUpFixture.cs. Add E2E coverage that builds a source from singleton and multi-file manifests, then verify the expected packages are present in the resulting index.