diff validate: redirect check silently skips all files when docset.yml is at repo root

## Summary

The `diff validate` redirect health check (added in #1340) silently passes on repositories where `docset.yml` lives at the git root (e.g. `elastic/docs-content`). Deleted and renamed files are never checked against `redirects.yml`, so the CI step always exits 0 regardless of whether redirects are missing.

## Root cause

`GetDocsetPathFromKnownLocations` correctly finds `docset.yml` at the repo root, so `DocumentationSourceDirectory` is set to the git root itself. This means:

```csharp
var relativePath = Path.GetRelativePath(root.FullName, buildContext.DocumentationSourceDirectory.FullName);
// root == docSourceDir → relativePath = "."
```

`IntegrationGitRepositoryTracker` then builds its lookup prefix as:

```csharp
LookupPath = $"{"." .Trim(['/', '\\'])}/"; // = "./"
```

But `DELETED_FILES` from the workflow contains paths like `solutions/search/ai-search/ai-search.md` — no leading `./` — so **every file is filtered out** and the loop finds nothing to validate.

## Impact

Confirmed against `elastic/docs-content` PR [elastic/docs-content#5567](https://github.com/elastic/docs-content/pull/5567), which deleted `solutions/search/ai-search/ai-search.md` with no redirect entry in `redirects.yml`. The `Validate redirect rules` CI step ran and reported success. The missing redirect was only caught later and fixed manually in a follow-up PR ([elastic/docs-content#7859](https://github.com/elastic/docs-content/pull/7859)), after the broken URL had already been live.

## Proposed fix

In `IntegrationGitRepositoryTracker`, treat `.` or empty `lookupPath` as "no prefix filter" (match all files):

```csharp
// Before
private string LookupPath { get; } = $"{lookupPath.Trim(['/', '\\'])}/";

// After
private string LookupPath { get; } = lookupPath.Trim(['/', '\\']) is "." or ""
? string.Empty
: $"{lookupPath.Trim(['/', '\\'])}/";
```

And update the filter predicate:

```csharp
.Where(f => string.IsNullOrEmpty(LookupPath) || f.StartsWith(LookupPath, OrdinalIgnoreCase))
```

The same fix should be applied symmetrically in `LocalGitRepositoryTracker` if it uses the same prefix logic.

## Affected repos

Any docs repo with `docset.yml` at the git root rather than in a `docs/` subdirectory. `elastic/docs-content` is the primary example.

Start with IntegrationGitRepositoryTracker and inspect how LookupPath is built and applied when DocumentationSourceDirectory is the git root. Check LocalGitRepositoryTracker for the same prefix logic, then run the diff validate workflow against root-level and nested docset paths; done means deleted or renamed files are checked for redirect entries in both layouts.

Written by the indexing model from the issue text.