fix(security): sanitize error responses in OpenAI-compatible APIs

## Why

The OpenAI-compatible API endpoints (`openai-http.ts`, `openresponses-http.ts`)
were returning raw `String(err)` in error responses. This can leak sensitive
internal information to HTTP clients:

- Stack traces revealing code structure
- File paths exposing server directory layout
- Internal error messages from dependencies

This is the same class of vulnerability that was fixed in #2387 for
`server-http.ts`, but these OpenAI-compatible endpoints were missed.

## What

Replace `String(err)` with safe error messages:

- **500 errors (api_error)**: Return generic "Internal server error"
- **400 errors (invalid_request_error)**: Return `err.message` only
  (no stack trace), with fallback to "Invalid request"

## Changes

- `src/gateway/openai-http.ts`: 2 catch blocks sanitized
- `src/gateway/openresponses-http.ts`: 4 catch blocks sanitized

src/gateway/openai-http.ts

@@ -239,9 +239,9 @@ export async function handleOpenAiHttpRequest(
         ],
         usage: { prompt_tokens: 0, completion_tokens: 0, total_tokens: 0 },
       });
-    } catch (err) {
+    } catch {
       sendJson(res, 500, {
-        error: { message: String(err), type: "api_error" },
+        error: { message: "Internal server error", type: "api_error" },
       });
     }
     return true;
@@ -361,7 +361,7 @@ export async function handleOpenAiHttpRequest(
           ],
         });
       }
-    } catch (err) {
+    } catch {
       if (closed) return;
       writeSse(res, {
         id: runId,
@@ -371,7 +371,7 @@ export async function handleOpenAiHttpRequest(
         choices: [
           {
             index: 0,
-            delta: { content: `Error: ${String(err)}` },
+            delta: { content: "Error: Internal server error" },
             finish_reason: "stop",
           },
         ],

src/gateway/openresponses-http.ts

@@ -435,7 +435,7 @@ export async function handleOpenResponsesHttpRequest(
     }
   } catch (err) {
     sendJson(res, 400, {
-      error: { message: String(err), type: "invalid_request_error" },
+      error: { message: err instanceof Error ? err.message : "Invalid request", type: "invalid_request_error" },
     });
     return true;
   }
@@ -452,7 +452,7 @@ export async function handleOpenResponsesHttpRequest(
     toolChoicePrompt = toolChoiceResult.extraSystemPrompt;
   } catch (err) {
     sendJson(res, 400, {
-      error: { message: String(err), type: "invalid_request_error" },
+      error: { message: err instanceof Error ? err.message : "Invalid request", type: "invalid_request_error" },
     });
     return true;
   }
@@ -565,13 +565,13 @@ export async function handleOpenResponsesHttpRequest(
       });
 
       sendJson(res, 200, response);
-    } catch (err) {
+    } catch {
       const response = createResponseResource({
         id: responseId,
         model,
         status: "failed",
         output: [],
-        error: { code: "api_error", message: String(err) },
+        error: { code: "api_error", message: "Internal server error" },
       });
       sendJson(res, 500, response);
     }
@@ -844,7 +844,7 @@ export async function handleOpenResponsesHttpRequest(
           delta: content,
         });
       }
-    } catch (err) {
+    } catch {
       if (closed) return;
 
       finalUsage = finalUsage ?? createEmptyUsage();
@@ -853,7 +853,7 @@ export async function handleOpenResponsesHttpRequest(
         model,
         status: "failed",
         output: [],
-        error: { code: "api_error", message: String(err) },
+        error: { code: "api_error", message: "Internal server error" },
         usage: finalUsage,
       });