add rastervision check command to validate config URIs

[Pranava-Kumar]

Summary

Adds rastervision check CLI command that validates all URIs in a PipelineConfig without running the pipeline. Users can verify their config files are accessible before kicking off a long-running job, avoiding late-stage failures from bad S3 paths or missing files.

Changes

• collect_uris() utility (file_system/utils.py): Recursively walks a pydantic BaseModel tree to find all fields ending in uri/uris. Handles nested models, lists, dicts, and None values.
• check CLI subcommand (cli.py): Loads a config module via get_configs(), collects all URIs, checks each with file_exists(), and reports results with color-coded output and a summary line.
• 19 tests (tests/pipeline/test_cli_check.py): 12 unit tests for collect_uris() and 7 integration tests for the CLI command using CliRunner.

Usage

rastervision check path/to/config.py -a root_uri /tmp/output

Output:

============================================================
  PipelineConfig [0]
============================================================
  [✓] dataset.train_scenes[0].raster_source.uris[0]: /data/img.tif
  [✓] dataset.train_scenes[0].aoi_uris[0]: /data/aoi.geojson
  [ ] dataset.validation_scenes[0].raster_source.uris[0]: /data/missing.tif

  Summary: 3 URI(s) checked, 2 found, 1 missing

Related Issue

Closes #1154