Support public HK snapshot artifact source

[Pigbibi]

Summary

• add data_source_mode to the HK low-vol snapshot bridge workflow
• default the bridge to public_yfinance_staging, while preserving LongBridge OpenAPI mode
• only retrieve LongBridge secrets for LongBridge-generated inputs
• update HK runtime docs for broker-independent snapshot artifact generation

Verification

• .venv/bin/python -m ruff check .
• .venv/bin/python -m pytest -q
• ruby -e "require 'yaml'; YAML.load_file('.github/workflows/build-hk-low-vol-snapshot-artifacts.yml'); puts 'yaml ok'"

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Install yfinance for the public data mode

When the workflow runs with the new default data_source_mode=public_yfinance_staging, this installs hk-equity-snapshot-pipelines[public-data], but the current HkEquitySnapshotPipelines pyproject.toml only defines test and longbridge extras, while the public snapshot provider raises if yfinance is unavailable. Pip will only warn about the unknown extra and install the base package, so the default public run reaches scripts/build_low_vol_dividend_public_factor_snapshot.py and fails before producing artifacts.

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