docs(api-reference): add context override + 403 to /api/chat/workflow

Close two contract gaps surfaced by the open-agents diff (PR comment):

- Add optional `context.contextLimit: number` to ChatWorkflowRequest
  so clients that already send `context` against open-agents' /api/chat
  port over unchanged.

- Document 403 in the response list — separate from 404 so callers can
  distinguish "session/chat doesn't exist" from "session/chat exists
  but the API key's account doesn't own it" (the ownership check the
  Bearer auth model requires).

Error response shape (`{status, message}` via ChatStreamErrorResponse)
intentionally kept — matches existing api convention rather than
open-agents' `{error, issues?}` shape.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sweetmantech

api-reference/openapi/research.json

@@ -793,8 +793,18 @@
               }
             }
           },
+          "403": {
+            "description": "The API key's account does not own the referenced `sessionId` or `chatId`.",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ChatStreamErrorResponse"
+                }
+              }
+            }
+          },
           "404": {
-            "description": "Session or chat not found, or not owned by the API key's account.",
+            "description": "Session or chat not found.",
             "content": {
               "application/json": {
                 "schema": {
@@ -3244,6 +3254,16 @@
             "type": "string",
             "format": "uuid",
             "description": "UUID of the session that owns both the chat and the sandbox. The session must have a non-null `sandbox_state` — call [POST /api/sandbox](/api-reference/sandbox/create) first if not."
+          },
+          "context": {
+            "type": "object",
+            "description": "Optional per-prompt context overrides forwarded into the agent loop.",
+            "properties": {
+              "contextLimit": {
+                "type": "integer",
+                "description": "Upper bound on the number of tokens / messages of conversation history the agent should consider for this prompt. Overrides the model's default for this call only."
+              }
+            }
           }
         }
       },