Test coverage: JsonSyncable multi-client file fan-out and malformed-file tolerance

Follow-up from the test-suite review (batches merged as #92–#96). Existing JsonSyncable tests assert only file existence / line count and use a single client id, so the multi-client format behavior and error tolerance are untested.

Gaps

Per-client file fan-out — commits are grouped by ClientId into separate client_<id>.jsonl files, and GetSyncState aggregates heads across them. Every test uses one ClientId, so this is unexercised.

https://github.com/sillsdev/harmony/blob/03f609f11a560433ce4a7ecd598a21921b324add/src/SIL.Harmony/JsonSyncable.cs#L50-L78

Malformed / blank / truncated files — empty file yields nothing, blank lines are skipped, null deserialize is skipped. A crash mid-write leaves a truncated trailing line; no test confirms it is tolerated rather than making the whole client file unreadable.

https://github.com/sillsdev/harmony/blob/03f609f11a560433ce4a7ecd598a21921b324add/src/SIL.Harmony/JsonSyncable.cs#L116-L133

ClientIdForFile on a non-GUID filename — a stray client_*.jsonl with an unparseable id would throw from Guid.Parse inside the parallel loop; behavior is undefined by tests.

https://github.com/sillsdev/harmony/blob/03f609f11a560433ce4a7ecd598a21921b324add/src/SIL.Harmony/JsonSyncable.cs#L110-L114

Content round-trip — no test reads a commit back and verifies ChangeEntities / HybridDateTime / Metadata survived; a field-drop regression passes today.

Suggested tests

• AddRange_MultipleClients_WritesSeparateFilesAndAggregatesSyncState
• AddRangeThenGetChanges_RoundTripsCommitContent
• ReadAllCommits_SkipsBlankLinesAndTruncatedTrailingLine
• GetSyncState_IgnoresOrHandlesUnparseableClientFile

Why it matters

JSONL files are the on-disk sync format. Real projects have many clients (fan-out is load-bearing) and interrupted writes are expected in practice; today a serialization or truncation regression would go undetected.

Start by locating the existing JsonSyncable tests and read src/SIL.Harmony/JsonSyncable.cs at the cited file fan-out, deserialization, and ClientIdForFile sections. Add the four suggested test cases covering multiple clients, content round-tripping, blank or truncated lines, and an unparseable client filename, then run the relevant test suite and verify all cases pass.