Add the runnable video generation pipeline

### Feature Summary

Step 9 of the operator demo video generator wires all previous components together into a runnable tool.

The previous steps added the pieces (the automation layer, the controllers, the config framework, and the demo data) but nothing calls them. This step adds the generator that turns demo data into per-operator scripts, the runner that executes a script and records it, and the command that drives both. This is the step that makes the feature usable.

### Proposed Solution or Design

Add the components needed to make the operator demo video generator runnable.

1. OperatorScriptGenerator:
Generates one script per operator from its metadata and sample values. It decides which workflow template to use, how to connect the operator, and whether dataset or code-editor steps are needed.

`sbt "Docs/runMain org.apache.texera.docs.scripts.OperatorScriptGenerator"`

2. VideoRunner
Runs each generated script in Playwright, manages the browser session, records the demo video, and saves debugging information if a run fails.

3. VideoGeneratorMain
Provides the CLI entry point and supports generating videos for all operators, a single operator, or one operator group.

```
sbt "Docs/runMain org.apache.texera.docs.VideoGeneratorMain --names=BarChart"
sbt "Docs/runMain org.apache.texera.docs.VideoGeneratorMain --group=Visualization"
```

4. Contributor guide
Add documentation for running the generator and validator, adding a new operator demo, and setting up the required local environment.

### Affected Area

_No response_

Start by locating the existing automation layer, controllers, config framework, and demo data, then trace the proposed OperatorScriptGenerator, VideoRunner, and VideoGeneratorMain entry points. Run the documented sbt commands for BarChart and the Visualization group to understand the expected workflows. Done means scripts are generated, videos and failure diagnostics are recorded, the CLI supports all three scopes, and the contributor guide covers setup, validation, and new demos.

Written by the indexing model from the issue text.