fix(mobile): retry legacy upload after transport reset

[kendraferguson14]

Summary

• retry the legacy media upload alias when the standard upload route fails with a ClientException or SocketException
• preserve user cancellation before attempting the compatibility retry
• cover the observed connection-reset regression with a focused mobile test

Older self-hosted relays can reject the standard upload route before consuming the streamed request body. Dart then surfaces the half-closed request as “Connection reset by peer” instead of exposing the HTTP 404/405 that triggers the existing fallback. Blossom uploads are content-addressed and idempotent, so retrying the legacy alias is safe even if the first response was lost after storage.

Related issue

No exact duplicate found. The closest upload reliability work is #3766, which covers cancellation rather than early-response transport resets.

Testing

• flutter test test/shared/relay/media_upload_test.dart — 36 passed
• flutter analyze lib/shared/relay/media_upload.dart test/shared/relay/media_upload_test.dart — no issues
• reproduced against a self-hosted relay that returned 405 on the standard upload route; the mobile-visible failure was ClientException with SocketConnection reset by peer