.Net: fix: Remove obsolete ITextSearch vector store integration tests (#13535)

# fix: Remove obsolete ITextSearch vector store integration tests

These tests were part of old MEVD integration tests removed in #13484.
The fixtures they depend on were intentionally deleted as part of
architectural cleanup.

Coverage for vector stores continues via VectorData.ConformanceTests.
ITextSearch interface testing continues via web connector tests.

Resolves build failures from merge commit 948ffd6.

### Motivation and Context

**1. Why is this change required?**

The feature branch `feature-text-search-linq` is experiencing build
failures after merging main (commit 948ffd6). CI/CD fails on all
platforms with compilation errors preventing the branch from being
merged to main.

**2. What problem does it solve?**

Resolves 3 compilation errors:
- `QdrantVectorStoreFixture` not found
- `AzureAISearchVectorStoreFixture` not found
- Test files reference deleted fixtures

**3. What scenario does it contribute to?**

This fix enables the feature-text-search-linq branch to build
successfully and proceed with the final merge to main. It aligns the
branch with architectural changes made in #13484 where old MEVD
integration tests were removed.

**4. Related issues:**

- Failed build:
https://github.com/microsoft/semantic-kernel/actions/runs/21822919939/job/62960593139
- Related to #13484 (MEVD integration test cleanup)
- Part of #13384 (feature branch PR)

### Description

**Root Cause:**

PR #13484 (merged Jan 29, 2026) removed old MEVD integration tests from
main including test fixtures. PR #13179 (merged Oct 29, 2025) added
ITextSearch integration tests to the feature branch that depend on those
fixtures. When main was merged into the feature branch (commit 948ffd6),
the test files were kept but the fixtures are gone, causing compilation
errors.

**Changes:**

Removes 3 obsolete test files (328 deletions):
-
`dotnet/src/IntegrationTests/Connectors/Memory/AzureAISearch/AzureAISearchTextSearchTests.cs`
-
`dotnet/src/IntegrationTests/Connectors/Memory/Qdrant/QdrantTextSearchTests.cs`
-
`dotnet/src/IntegrationTests/Connectors/Memory/InMemory/InMemoryVectorStoreTextSearchTests.cs`

**Rationale:**

These tests should be removed because:
1. They test the deprecated `ITextSearch` interface (marked
`[Obsolete]`), not the modern `ITextSearch<TRecord>`
2. They depend on fixtures intentionally deleted in #13484 architectural
cleanup
3. Test coverage already exists elsewhere

**Test Coverage Maintained Through:**

- Vector store functionality:
`dotnet/test/VectorData/*Connector*.ConformanceTests/`
- VectorStoreTextSearch wrapper:
`dotnet/src/SemanticKernel.UnitTests/Data/`
- ITextSearch interface: `dotnet/src/IntegrationTests/Plugins/Web/`
(Bing, Google, Tavily tests)

**Impact:**

No impact on the 6 PRs that comprise this feature branch (#13175,
#13179, #13188, #13190, #13191, #13194). All implement the modern
`ITextSearch<TRecord>` interface and are properly tested.

### Contribution Checklist

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
(N/A - file removal only)
- [x] I didn't break anyone 😄 (removes broken tests, no
functionality changes)

Co-authored-by: Alexander Zarei <alzarei@users.noreply.github.com>

Author: alzarei