fix(matrix): handle encrypted media in E2EE rooms

[elkimek]

Summary

• Encrypted media (images, audio, video, files) in E2EE Matrix rooms were silently dropped
• matrix-nio emits RoomEncryptedImage/Audio/Video/File for these — distinct from MegolmEvent (encrypted text) and RoomMessageImage (unencrypted media)
• No callbacks were registered for these event types, so the gateway never processed them

Changes

• Register event callbacks for all four RoomEncrypted* media types
• Extend isinstance checks in _on_room_message_media to match both encrypted and unencrypted variants
• Decrypt attachment data after download using nio.crypto.decrypt_attachment before caching

Test plan

• Send an image in an E2EE Matrix room — bot receives and processes it
• Vision analysis works on decrypted image (valid JPEG/PNG, not encrypted bytes)
• Unencrypted rooms still work as before
• Existing test suite: 6887 passed, 0 new failures
• Audio/video/file attachments in E2EE rooms (same code path, not yet tested)

Platforms tested

• Linux (Ubuntu 24.04, Python 3.12)

🤖 Generated with Claude Code

[teknium1]

Merged via PR #5271 using #3140's implementation which is more comprehensive — handles all media types (not just images), prevents ciphertext URL fallback, includes 355 lines of tests. Your approach was correct for the image case. Thanks!