feat(gmail): add --body-format flag to messages search

[alexknowshtml]

Summary

Adds --body-format to gmail messages search for controlling which MIME part is returned when --include-body is used.

• --body-format text (default) — preserves existing behavior, returns plain text part (falls back to HTML if no plain text)
• --body-format html — prefers the HTML part (falls back to plain text if no HTML)

Adds bestBodyHTML() helper to complement bestBodyText().

Motivation

Newsletters and rich emails often have a minimal plain text part (or just "view in browser") while all meaningful content is in the HTML part. This flag makes it possible to retrieve that content without custom workarounds.

Usage

gog gmail messages search "from:newsletter@example.com" --include-body --body-format html

Test plan

• --body-format text returns existing behavior (plain text, HTML fallback)
• --body-format html returns HTML part when present
• --body-format html falls back to plain text when no HTML part exists
• Omitting --body-format defaults to text behavior
• Invalid value is rejected by kong enum validation

🤖 Generated with Claude Code

[steipete]

Thanks @alexknowshtml, landed on main as d08771e plus maintainer follow-up 6e0af2c.

Before landing I kept the feature, removed the accidental byline-cache churn via your second commit, and added:

• explicit body-format constants/helper plumbing instead of a variadic test dodge
• regression coverage for default text body selection, HTML preference, and plain fallback
• README, spec, and changelog coverage

Verified:

• focused Gmail body-format tests
• make ci
• live Gmail smoke on steipete@gmail.com with --body-format text and html; only counts/body lengths were printed, and the HTML path returned an HTML-looking body.