[Story] Edit a media caption

[pixlwave]

Description

• As a user
• I want to be able to edit the caption on a media file that I have sent
• So that I can hide my inability to type correctly.

Acceptance criteria

• Editing a caption should not change the image, only the text should be updated.
• If markdown was used to send the caption, it should be restored to the composer to help fix syntax errors.

Size estimate

None

Dependencies

• Rust: Editing a caption would happen using the edit() function on the timeline, using EditedContent::MediaCaption for the media event — this will fail if the underlying event wasn't a room message of a media type.

Out of scope

• Nothing

Open questions

### Questions

Subtasks

### Android
- [ ] https://github.com/element-hq/element-x-android/issues/3859

### iOS
- [ ] https://github.com/element-hq/element-x-ios/issues/3509

### Rust
- [ ] https://github.com/matrix-org/matrix-rust-sdk/pull/4277
- [ ] https://github.com/matrix-org/matrix-rust-sdk/issues/4302

### Other