* feat: add order_by and order params to /v1/conversations list endpoint [LET-7628]
Added sorting support to the conversations list endpoint, matching the pattern from /v1/agents.
**API Changes:**
- Added `order` query param: "asc" or "desc" (default: "desc")
- Added `order_by` query param: "created_at" or "last_run_completion" (default: "created_at")
**Implementation:**
**created_at ordering:**
- Simple ORDER BY on ConversationModel.created_at
- No join required, fast query
- Nulls not applicable (created_at always set)
**last_run_completion ordering:**
- LEFT JOIN with runs table using subquery
- Subquery: MAX(completed_at) grouped by conversation_id
- Uses OUTER JOIN so conversations with no runs are included
- Nulls last ordering (conversations with no runs go to end)
- Index on runs.conversation_id ensures performant join
**Pagination:**
- Cursor-based pagination with `after` parameter
- Handles null values correctly for last_run_completion
- For created_at: simple timestamp comparison
- For last_run_completion: complex null-aware cursor logic
**Performance:**
- Existing index: `ix_runs_conversation_id` on runs table
- Subquery with GROUP BY is efficient for this use case
- OUTER JOIN ensures conversations without runs are included
**Follows agents pattern:**
- Same parameter names (order, order_by)
- Same Literal types and defaults
- Converts "asc"/"desc" to ascending boolean internally
🐾 Generated with [Letta Code](https://letta.com)
Co-Authored-By: Letta <noreply@letta.com>
* chore: order
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Co-authored-by: Letta <noreply@letta.com>
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