fix(media): handle app card cover fallback

[SeniorZhai]

No description provided.

[Copilot]

Pull request overview

This PR improves media viewing behavior for App Card messages by correctly resolving cover URLs when the cover is provided via the nested cover.url field (instead of only cover_url), and by preventing the media pager from proceeding when the target “media message” can’t be found/doesn’t qualify.

Changes:

• Add AppCardData.hasCover and use it to detect whether an App Card should be treated as having media cover.
• Update MessageItem.appCardCoverUrl() to fall back to cover.url when cover_url is blank/empty.
• Harden media pager indexing/loading to handle “not found” cases and filter out App Cards without a usable cover.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
app/src/test/java/one/mixin/android/vo/AppCardDataTest.kt	Adds a unit test to ensure nested cover.url is treated as a valid cover source.
app/src/main/java/one/mixin/android/vo/MessageItem.kt	Adds isAppCardWithCover() helper for cover-aware App Card filtering.
app/src/main/java/one/mixin/android/vo/AppCardData.kt	Introduces hasCover and updates appCardCoverUrl() to ignore blank strings and fall back properly.
app/src/main/java/one/mixin/android/ui/media/pager/MediaPagerActivity.kt	Stops media paging flow early when the indexed target can’t be located.
app/src/main/java/one/mixin/android/repository/ConversationRepository.kt	Validates media message eligibility (exclude-live constraints + App Card cover presence) before indexing/returning media messages.

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