fix(sdk): classify not-found only by explicit FILE_NOT_FOUND code

Address review feedback: isFileNotFound() previously treated any HTTP
404 as not-found. A 404 whose body cannot be parsed is mapped to
UNEXPECTED_RESPONSE and may signal a real endpoint/routing regression;
downgrading those to DEBUG would hide genuine failures.

Restrict detection to the explicit SandboxError.FILE_NOT_FOUND code
(which the execd server returns for missing files) and add a
regression test covering a bare 404 + UNEXPECTED_RESPONSE.

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

Author: hittyt

sdks/sandbox/kotlin/sandbox/src/main/kotlin/com/alibaba/opensandbox/sandbox/infrastructure/adapters/converter/ExceptionConverter.kt

@@ -43,11 +43,16 @@ import com.alibaba.opensandbox.sandbox.api.execd.infrastructure.ServerException
  * Returns `true` when this throwable represents an expected "file or directory does not exist"
  * outcome rather than a genuine failure.
  *
+ * Detection is intentionally restricted to the explicit [SandboxError.FILE_NOT_FOUND] server
+ * error code rather than a bare HTTP 404. A 404 whose body cannot be parsed is mapped to
+ * [SandboxError.UNEXPECTED_RESPONSE] and may indicate a real endpoint/routing/configuration
+ * regression, which must stay loud (ERROR) instead of being silently downgraded.
+ *
  * Callers (and the adapters themselves) use this to avoid treating a missing file as an error,
  * e.g. logging it at ERROR level with a full stack trace, which is just noise for a perfectly
  * normal control-flow case such as polling for a not-yet-created file.
  */
-fun Throwable.isFileNotFound(): Boolean = this is SandboxApiException && (error.code == SandboxError.FILE_NOT_FOUND || statusCode == 404)
+fun Throwable.isFileNotFound(): Boolean = this is SandboxApiException && error.code == SandboxError.FILE_NOT_FOUND
 
 fun Exception.toSandboxException(): SandboxException {
     return when (this) {

sdks/sandbox/kotlin/sandbox/src/test/kotlin/com/alibaba/opensandbox/sandbox/infrastructure/adapters/service/FilesystemAdapterTest.kt

@@ -121,6 +121,20 @@ class FilesystemAdapterTest {
         assertFalse(exception.isFileNotFound())
     }
 
+    @Test
+    fun `isFileNotFound is false for a 404 without an explicit FILE_NOT_FOUND code`() {
+        // A 404 whose body could not be parsed is mapped to UNEXPECTED_RESPONSE. It may indicate a
+        // real endpoint/routing regression, so it must NOT be downgraded to a not-found condition.
+        val exception =
+            SandboxApiException(
+                message = "Failed to read file. Status code: 404",
+                statusCode = 404,
+                error = SandboxError(SandboxError.UNEXPECTED_RESPONSE),
+            )
+
+        assertFalse(exception.isFileNotFound())
+    }
+
     @Test
     fun `isFileNotFound is false for non-sandbox exceptions`() {
         assertFalse(RuntimeException("boom").isFileNotFound())