Improve external snippet validations and extraction tests

[coldice]

• Add two snippet validators plus tests.
• Extend tests/test_generate_external_snippets.py to cover generateOutputDocs.js location types, indent, transforms, CLI flags, and error paths.
• Wire npm scripts: validate-snippet-files, validate-snippet-sources.

validate-snippet-files
npm run validate-snippet-files
Checks docs-main page imports vs snippet files. Reports missing and orphan snippets. Writes snippets-missing.log / snippets-orphan.log.

usage: validate_snippet_files.py [-h] [--quiet] [--no-fail]
                                 [--output-path OUTPUT_PATH]
                                 [--delete-orphan-snippets]
  --quiet                   Only print errors (missing or orphan snippets)
  --no-fail                 Exit 0 even when missing or orphan snippets are found
  --output-path PATH        Directory for snippets-missing.log and
                            snippets-orphan.log (default: repo root)
  --delete-orphan-snippets  After writing snippets-orphan.log, delete orphan
                            snippet files

validate-snippet-sources
npm run validate-snippet-sources
Checks remote snippet configs against source-repo checkouts (lines, stringMarker, jsonIndex, regexWrap, fullFile). Writes snippet-source-errors.log.

usage: validate_snippet_sources.py [-h] [--quiet] [--no-fail]
                                   [--output-path OUTPUT_PATH]
                                   [--source-dir SOURCE_DIR]
                                   [repo]

  repo               Only validate this remote snippet repo
                     (e.g. daml, canton, splice).
                     Omit to check every repo in remote-snippet-lists.json
  --quiet            Only print source validation errors
  --no-fail          Exit 0 even when source validation errors are found
  --output-path PATH Directory for snippet-source-errors.log
                     (default: repo root)
  --source-dir PATH  Path to the git checkout for the selected repo.
                     Required with a repo name if autodiscovery cannot
                     find a unique checkout

[Jatinp26]

@danielporterda could you please the changes as well? before I merge/review it.