Fix Solution Explorer repo grouping, filtering, and theming for merged VMR sites

[tannergooding]

Bundles the "make the site work as expected" fixes for multi-repo / VMR merged sites (the kind source.dot.net produces), so a merged index correctly scopes, groups, and themes each repo -- including the VMR case where one repo (dotnet/dotnet) physically contains many others under src/.

Repo grouping and search scope

• Tag each project with its full repo chain (outermost -> innermost), persisted in ProjectInfo.txt (RepoChain=) and the Assemblies.txt 6th field. A parent repo (e.g. dotnet/vmr) now includes its nested sub-repos (dotnet/subx, dotnet/suby) in both search scope (Query.RepoMatches is ancestor-or-self) and the Solution Explorer, displayed nested under the parent. Selecting a sub-repo still scopes to just that sub-repo.
• /repoPath-mapped repos now get their own repo-filter entry, matching /repo behavior -- previously a /repoPath mapping (e.g. dotnet/arcade under dotnet/dotnet) had no filter option.

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Duplicate repo handling

• When the same repo is passed twice with different URLs, emit a non-critical warning naming the winning vs losing URL instead of silently picking one.

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Solution Explorer UI

• Fix multi-solution repos rendering with the light theme regardless of the active theme.
• Repo filtering is ancestor-aware, and grouping folders a filter leaves empty (including MSBuild solution folders, which carry no repo path) are now hidden instead of lingering as hollow headers.
• Repo/solution grouping bands now render full-width at every nesting depth, with only the text/arrow carrying the depth indent -- nested bands previously staircased inward and stopped short of the pane edge.

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Sample / tests

• Extends the sample (HtmlGenerator launchSettings + new TestCode/Vmr fixture with SubRepoX/SubRepoY under a VMR root, plus a second solution in RepoB) to cover the nested-repo and duplicate-URL scenarios so they can be validated locally.
• All tests green: 184 HtmlGenerator + 40 SourceIndexServer.

[tannergooding]