## What this PR does

Removes the self-concatenation of the vision model response in the video
parsing path, so each generated video description is tokenized and
indexed exactly once.

A focused regression test exercises the public `picture.chunk` video
path with a mocked vision model and asserts that the returned
description is passed to `tokenize` once without duplication.

## Root cause

The original video parsing implementation used:

```python
ans += "\n" + ans
tokenize(doc, ans, ...)
```

This duplicates the same model response. The adjacent image path
combines two distinct values (`OCR text + vision description`); the
video path has only the model response, so concatenating it with itself
is an unintended copy/paste error from that image logic.

## Impact

Before this fix, every successfully parsed video stored repeated text,
increasing token and embedding input and potentially distorting indexed
chunk content and retrieval scoring.

## Compatibility

The change affects only the video branch in `rag/app/picture.py`. Image
parsing, model invocation, prompts, callbacks, and error handling remain
unchanged.

## Validation

- `pytest --confcutdir=test/unit_test/rag/app
test/unit_test/rag/app/test_picture_video.py -q`: 1 passed
- Ruff check: passed
- Ruff format check for the new test: passed
- `git diff --check`: passed

Closes #16846.

---------

Co-authored-by: openhands <openhands@all-hands.dev>