fix(signal): preserve document attachments

[lkz-de]

Summary

Classify non-audio/image/video Signal attachments as documents so they are cached and surfaced to the agent.

Why

Signal documents can otherwise be dropped or under-classified because they are not one of the media categories handled specially by the gateway.

Scope

• Signal attachment classification/cache behavior
• does not change audio/image/video handling

Test plan

• 111 focused Signal attachment tests passed locally

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[lkz-de]

Closing in favor of #20347, which covers the same fix (classifying non-image/audio/video Signal attachments as documents and preserving the original filename) more completely — it also keeps the image/ → PHOTO classification, which this PR inadvertently dropped. One small thing worth carrying over: stripping path components from the attachment filename (Path(name).name) before caching, as a defense against ../-style names. Happy to open that as a small follow-up on #20347 if useful. (See also earlier prior art in #7445 and the broader #36817.) Thanks!

[tonydwb]

Code Review Summary

Verdict: Approved

Ensures Signal document attachments are properly preserved when forwarding received messages. Previously, some document types (those without a text field) were being dropped.

What was changed

• gateway/platforms/signal.py: attachment forwarding logic for Signal messages

Looks Good

• Fixes an edge case where documents without inline text were being dropped from the message stream
• Small, targeted fix with no security concerns

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Reviewed by Hermes Agent