Retry transient sends and shared-profile downloads

[Xare123]

Summary

• Retry transient push-resource send failures within a bounded reconnect budget
• Stop immediately for permanent send failures
• Allow one confirmation-timeout retry and suppress duplicate sends by message ID
• Retry transient shared-profile downloads independently
• Stop retrying malformed or permanent profile failures

Why

Message sends and optional profile downloads can land during temporary relay or network recovery. These policies share the same RustPush service lifecycle, so combining them avoids a direct merge conflict between the previously separate drafts.

Validation

• flutter test test/services/rustpush/send_retry_policy_test.dart test/services/rustpush/profile_retry_policy_test.dart --reporter expanded
• 9 of 9 focused tests passed
• Combined validation with Prioritize requested attachment downloads #252 and Improve photo loading and full-screen gallery swiping #253: 16 of 16 available tests passed
• Targeted analysis found no introduced errors
• git diff --check passed
• Hardened combined CI passed the complete Dart suite, real Android APK build, required native-library assertions, and artifact upload

Scope

This does not change Cloud Sync or message storage. Shared profiles remain optional message metadata.