fix(gmail): retrieve externally stored message text bodies

[rudycelekli]

Gmail can return a text message body through body.attachmentId with absent or empty inline data. Thread reading previously returned an empty body for these messages. Retrieve external plain text and HTML bodies through the generated attachments API, then apply the existing decoding, HTML cleanup and credential redaction. Named file attachments remain attachment metadata.

Six regression/control cases exercise the real tool, OAuth client, generated Gmail SDK and HTTP transport against a local fixture with a synthetic token. Four cases fail before the fix and all six pass after; inline, empty and nested bodies remain readable, and retrieval errors surface. All 14 focused Google Workspace tests, full lint and TypeScript checks pass. Full pnpm check passes (96 files, 882 tests), and the exact source passes the unmodified author-fork Checks workflow, including runtime evaluation. Local pnpm build passes. The local build reported a Turbo cache disk-space warning at completion; completed output cleanup restored the dependency paths without altering source or installed packages. Live Gmail accounts were not accessed.

The unchanged Checks workflow completed successfully on exact signed head acdf235dc1cd3c7e83588c9a5519efe9a4725d81, including frozen installation, all six check tasks and the complete two-scenario / 13-gate runtime command with cleanup. The merge checkout tree matches the reviewed source. The local build exited 0; its Turbo cache ENOSPC warning and dependency restoration after completed build-output cleanup are retained. Local Docker cleanup remains blocked; it is not counted as passing. No workflow, dependency or runtime fixture changes.

AI assistance was used for investigation, implementation, tests and review under the submitting account. Signed DCO commit included.