feat: add conversation deletion endpoint (soft delete) [LET-7286] (#9230)

* feat: add conversation deletion endpoint (soft delete) [LET-7286]

- Add DELETE /conversations/{conversation_id} endpoint
- Filter soft-deleted conversations from list operations
- Add check_is_deleted=True to update/delete operations

Co-authored-by: Sarah Wooders <sarahwooders@users.noreply.github.com>

🤖 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* feat: add tests, update SDK and stainless for delete conversation

- Add 5 integration tests for DELETE conversation endpoint
- Run stage-api to regenerate OpenAPI spec and SDK
- Add delete method to conversations in stainless.yml

Co-authored-by: Sarah Wooders <sarahwooders@users.noreply.github.com>

🤖 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* test: add manager-level tests for conversation soft delete [LET-7286]

- test_delete_conversation_removes_from_list
- test_delete_conversation_double_delete_raises
- test_update_deleted_conversation_raises
- test_delete_conversation_excluded_from_summary_search

🤖 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

---------

Co-authored-by: letta-code <248085862+letta-code@users.noreply.github.com>
Co-authored-by: Letta <noreply@letta.com>
Co-authored-by: Sarah Wooders <sarahwooders@gmail.com>

fern/openapi.json

@@ -8922,6 +8922,52 @@
             }
           }
         }
+      },
+      "delete": {
+        "tags": ["conversations"],
+        "summary": "Delete Conversation",
+        "description": "Delete a conversation (soft delete).\n\nThis marks the conversation as deleted but does not permanently remove it from the database.\nThe conversation will no longer appear in list operations.\nAny isolated blocks associated with the conversation will be permanently deleted.",
+        "operationId": "delete_conversation",
+        "parameters": [
+          {
+            "name": "conversation_id",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string",
+              "minLength": 1,
+              "maxLength": 41,
+              "pattern": "^(default|conv-[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12})$",
+              "description": "The conversation identifier. Either the special value 'default' or an ID in the format 'conv-<uuid4>'",
+              "examples": [
+                "default",
+                "conv-123e4567-e89b-42d3-8456-426614174000"
+              ],
+              "title": "Conversation Id"
+            },
+            "description": "The conversation identifier. Either the special value 'default' or an ID in the format 'conv-<uuid4>'"
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Successful Response",
+            "content": {
+              "application/json": {
+                "schema": {}
+              }
+            }
+          },
+          "422": {
+            "description": "Validation Error",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/HTTPValidationError"
+                }
+              }
+            }
+          }
+        }
       }
     },
     "/v1/conversations/{conversation_id}/messages": {