[Bug] test_detect_file_encodings fails on Windows: cannot reopen an open NamedTemporaryFile

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### Dify version

`main` @ `b6824334cf` (source install, API 1.16.x)

### Cloud or Self Hosted

- [x] Self Hosted (Source)

### Steps to reproduce

On Windows, run:

```bash
uv run --project api pytest api/tests/unit_tests/core/rag/extractor/test_helpers.py::TestHelpers::test_detect_file_encodings
```

Failure:

```text
PermissionError: [Errno 13] Permission denied: 'C:\\Users\\...\\Temp\\tmpxxx.txt'
```

### ✔️ Expected Behavior

The test writes a small text file and detects its encoding; `detect_file_encodings` reopens the path with `charset_normalizer.from_path`. Passes on Linux/macOS.

### ❌ Actual Behavior

The test keeps `tempfile.NamedTemporaryFile(mode="w+t")` open inside the `with` block and calls `detect_file_encodings(temp_path)` before exiting it. On Windows a still-open `NamedTemporaryFile` cannot be reopened by another handle (documented platform difference), so the production helper raises `PermissionError`. This is a test-only portability bug; the fix is to close the file before detection (`delete=False` + explicit `os.unlink` in a `finally`).

Open api/tests/unit_tests/core/rag/extractor/test_helpers.py and inspect TestHelpers::test_detect_file_encodings, focusing on the NamedTemporaryFile lifecycle. Run the reported uv pytest command first, then verify the test closes and cleans up its temporary file before detection. Done means the test passes on Windows without PermissionError and continues to pass on other platforms.

Written by the indexing model from the issue text.