The Signal adapter inherited base class defaults for send_image_file(),
send_voice(), and send_video() which only sent the file path as text
(e.g. '🖼️ Image: /tmp/chart.png') instead of actually delivering the file
as a Signal attachment.
When agent responses contain MEDIA:/path/to/file tags, the gateway
media pipeline extracts them and routes through these methods by file
type. Without proper overrides, image/audio/video files were never
actually delivered to Signal users.
Extract a shared _send_attachment() helper that handles all file
validation, size checking, group/DM routing, and RPC dispatch. The four
public methods (send_document, send_image_file, send_voice, send_video)
now delegate to this helper, following the same pattern used by WhatsApp
(_send_media_to_bridge) and Discord (_send_file_attachment).
The helper also uses a single stat() call with try/except FileNotFoundError
instead of the previous exists() + stat() two-syscall pattern, eliminating
a TOCTOU race. As a bonus, send_document() now gains the 100MB size check
that was previously missing (inconsistency with send_image).
Add 25 tests covering all methods plus MEDIA: tag extraction integration,
method-override guards, and send_document's new size check.
Fixes#5105
5e88eb2ba0