[adithya-s-119]

Summary

Fixes several SMB playback and library-sync failure modes, with the unreachable/unreadable child-directory scan bug as the main one.

• Sync no longer aborts on one unreadable folder. Windows shares commonly contain entries such as System Volume Information that even a valid account cannot list. The BFS previously failed the entire scan there, which could leave a fresh SMB library empty and make playback look broken. Child directories that are denied, missing, or unsupported are now skipped; failure to list the configured root still fails loudly. Directory paths are de-duplicated and recursion depth is capped to stop junction/DFS loops.
• Cover art uses the pipelined SMB reader. read_file is a single READ and fails with FileTooLargeForSingleRead above the server's MaxReadSize (as small as 64 KiB on some NAS firmware). Folder art now loads on those servers.
• Shared downloads are isolated from UI callbacks. A throwing progress listener can no longer fail or cancel a transfer other callers are waiting on. The coalescing entry is released when the transfer itself finishes, not when the first waiter leaves, so a late call cannot start a duplicate download.
• Cache handling is more forgiving. A failed eviction pass no longer discards an already-completed download, a cache file that disappears during the LRU touch is treated as a miss, and a resync invalidates cached bytes when the listed file size changes. Cache extensions are normalized/derived from the remote name so callers that pass FLAC, flac, or no hint share one file.
• Gapless/prefetch failures are caught. A failed SMB download while preparing the next track now reports through the normal SMB failure path instead of throwing from an unawaited task.
• Connection resilience. SMB clients now use ClientConfig with auto-reconnect and a 15-second initial connect timeout instead of the 5-second, no-reconnect shorthand.

Related issue

Refs #291 (SMB playback). Does not claim to close it until the reporter can retest on the original Windows share.

Testing

• flutter analyze --no-fatal-infos: no errors or warnings (28 pre-existing infos, unchanged from main).
• flutter test test/services/smb_service_parse_test.dart test/services/smb_service_download_test.dart test/services/network_cache_service_test.dart test/services/remote_metadata_backfill_test.dart: 38 passed, including new coverage for throwing progress callbacks, extension normalization/derivation, and cache invalidation.
• Rust: the full cargo test --lib cannot run on this Windows host because the existing soundtouch-sys C++ build fails under the local GNU toolchain. I compiled the exact smb_api.rs in an isolated crate against smb2 0.27.2 and ran its tests: 7 passed (including the new child-directory/root distinction and client-config tests). CI's Linux cargo test --lib remains the authoritative full run.
• Full flutter test on this Windows host has pre-existing failures (layout overflows and platform-specific HTTP/cache tests). I confirmed the same failures on a clean main checkout; the SMB suite passes.

Checklist

• flutter analyze passes
• flutter test passes (SMB/network suites pass; full Windows run has pre-existing failures)
• Rust changes: cargo test --manifest-path rust/Cargo.toml passes (SMB module tests pass in isolation; full run blocked by existing soundtouch build issue on this host)
• Built and ran the app if the change touches a playback path (not run; no Android device/SMB share available here)
• Generated code regenerated and committed if bridge/drift/freezed models changed (none required)
• CHANGELOG.md updated for user-facing changes
• No secrets, keys, tokens, or machine-local files committed
• One change per PR; no unrelated reformatting in the same diff
• Hardware tested if the Rust audio engine or USB/DAC output changed (SMB module only; no audio-engine change)

[ultraelectronica]

Will be reviewed @adithya-s-119, thanks a lot for contributing!

[ultraelectronica]

Approved after local verification on Linux: flutter analyze --no-fatal-infos (28 pre-existing infos, no errors/warnings), targeted SMB/network tests (38 passed), full cargo test --lib (368 passed), and build_runner generated no stale outputs. The full Flutter suite has two unrelated pre-existing/local failures (webdav_sync_integration_test Isar Core version mismatch and a flaky Apple Music run). Changes match the existing SMB failure-handling and cache patterns.