[To3akaRin]

Provider messages define ContentElement.file_base64, but both Message and MessageChunk omit it when building a platform message chain. A Runner can return a valid inline file through message.delta or message.completed and the file is silently discarded before an adapter receives it.

Add the missing conversion to File(base64=..., name=...) in both paths. Preserve content order and the original payload, retain empty files, skip missing payloads consistently with adjacent media branches, and use the platform model's empty-name default when the optional filename is absent.

Refs langbot-app/LangBot#2733. LangBot's pinned SDK must adopt a release containing this fix before running installations receive the behavior.

Regression coverage

23 parameterized cases cover full and streaming messages, binary and empty files, Unicode/optional names, mixed content order, first-text prefixes, file-only prefixes, Host attachments, input immutability, and JSON round-trips through message.delta, message.completed, and run.completed.

The unchanged baseline produced 21 failures and 2 passes in these tests. After the fix, all 23 cases and the related 98-test entity/Runner set passed.

Validation

• Linux ARM64 / Python 3.12.12: 2,005 passed, 7 skipped, 84.75% coverage, above the repository's 84% gate.
• Installed-wheel CLI/package black-box suite: 5 passed.
• Full Ruff lint and format checks, scoped Mypy (38 files), and action-protocol consistency check passed.
• sdist and wheel builds passed; both contain the exact tested production source.
• Independent review found no blocking issues.

The macOS full run had 2,002 passes and one pre-existing failure: a runtime FD test reads Linux-only /proc/self/fd. The same failure reproduced in an unmodified 01533c4 worktree; the complete Linux run passed it. Environment-dependent skips remain visible in the validation record.

See the usage and regression guide. Actual WeCom delivery was not exercised; this change verifies SDK conversion and Runner serialization without changing platform APIs, wire fields, dependencies, or release versions.

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[To3akaRin]

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[To3akaRin]

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