Add BigQuery description length CI safeguard

[seamus-mckinsey]

Summary

• add a lightweight Python scanner for dbt schema YAML column descriptions using PyYAML
• add a pull_request/push workflow that installs PyYAML and runs the scanner for models, seeds, and snapshots
• keep the workflow warning-only for now because the repo currently has existing over-limit descriptions

Why

This addresses the BigQuery field-description length regressions surfaced in #1323 by adding a durable CI safeguard so new over-limit metadata does not slip in unnoticed.

Validation

• python3 -m unittest scripts/test_parse_ci_command.py
• python3 -m venv /tmp/tuva-pyyaml-check && /tmp/tuva-pyyaml-check/bin/python -m pip install pyyaml
• /tmp/tuva-pyyaml-check/bin/python -m unittest scripts/test_check_metadata_description_length.py
• /tmp/tuva-pyyaml-check/bin/python scripts/check_metadata_description_length.py --limit 1024
• ruby -e 'require "yaml"; Dir[".github/workflows/*.yml"].sort.each { |path| YAML.load_file(path) }'

Current violations

The scanner currently finds 10 existing descriptions over 1024 characters in input-layer YAML files. Once those are cleaned up, the workflow can be made required by removing continue-on-error.

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