Retry setup after transient source fetch failures

[PrzemyslawKlys]

Addresses #6981.

Summary

• make SourceShell.fetch() report success without discarding cached entries or refresh time on failure
• let Home Assistant retry a config entry when its first source fetch fails instead of loading sensors as unknown
• advance fetch-interval bookkeeping only after successful UI and YAML fetches, while still attempting every configured YAML source

This targets the stable master branch and is intentionally independent of the v3 source architecture.

Validation

• python -m pytest -q — 38 passed
• Ruff, format, codespell, bandit, and mypy hooks passed

CI dependency

The current base workflow installs unsupported Home Assistant 2023.7.3 under Python 3.10, so its validation job fails while importing DeviceInfo before these tests can run. #6987 aligns the base validation environment with the advertised Home Assistant 2023.12 minimum.