fix(trace messages): send ids (plural) to match backend field name

[Palash Shah (Palashio)]

Summary

The `trace messages` command was sending `body["id"]` (singular) when filtering by trace IDs, but the backend `BatchTraceMessagesRequestBody` struct expects `"ids"` (plural, `json:"ids"`). This caused the trace ID filter to be silently dropped — the backend unmarshaled the field to its zero value (nil slice) and returned all traces instead of the requested ones.

In `internal/cmd/message.go`:
```go
// Before (broken): backend ignores this field, returns unfiltered results
body["id"] = ids

// After (fixed): matches json:"ids" on BatchTraceMessagesRequestBody.Ids
body["ids"] = ids
```

The backend `POST /v2/traces/messages` reads from `body.Ids` (JSON field `"ids"`). With the singular key, every `--trace-ids` invocation silently returned unfiltered results rather than the specified traces. This is the specific failure mode causing the issues-agent's phase 1b screener to get wrong data when it calls `langsmith trace messages --trace-ids`.

Test Plan

• All existing `TestTraceMessages` unit tests pass
• Built binary and verified against production with all four sandbox command patterns:
  • `trace messages --project --limit 3 --last-n-minutes 360` → returns traces correctly
  • `trace messages --project --trace-ids "id1,id2" --limit 5 --last-n-minutes 360` → returns exactly 2 traces matching the specified IDs
  • `LANGSMITH_PROJECT= trace messages --project --trace-ids "id" --limit 5` → returns exactly 1 trace
  • Verified trace_ids in response match the requested IDs exactly

Release Note

Fix `langsmith trace messages --trace-ids` filter being silently ignored due to wrong JSON field name (`id` → `ids`).