fix(agent): retry on json.JSONDecodeError instead of treating it as a local validation error (#15107)

json.JSONDecodeError inherits from ValueError. The agent loop's
non-retryable classifier at run_agent.py ~L10782 treated any
ValueError/TypeError as a local programming bug and short-circuited
retry. Without a carve-out, a transient JSONDecodeError from a
provider that returned a malformed response body, a truncated stream,
or a router-layer corruption would fail the turn immediately.

Add JSONDecodeError to the existing UnicodeEncodeError exclusion
tuple so the classified-retry logic (which already handles 429/529/
context-overflow/etc.) gets to run on bad-JSON errors.

Tests (tests/run_agent/test_jsondecodeerror_retryable.py):
  - JSONDecodeError: NOT local validation
  - UnicodeEncodeError: NOT local validation (existing carve-out)
  - bare ValueError: IS local validation (programming bug)
  - bare TypeError: IS local validation (programming bug)
  - source-level assertion that run_agent.py still carries the carve-out
    (guards against accidental revert)

Closes #14782

tests/run_agent/test_jsondecodeerror_retryable.py

@@ -0,0 +1,87 @@
+"""Regression guard for #14782: json.JSONDecodeError must not be classified
+as a local validation error by the main agent loop.
+
+`json.JSONDecodeError` inherits from `ValueError`. The agent loop's
+non-retryable classifier at run_agent.py treats `ValueError` / `TypeError`
+as local programming bugs and skips retry. Without an explicit carve-out,
+a transient provider hiccup (malformed response body, truncated stream,
+routing-layer corruption) that surfaces as a JSONDecodeError would bypass
+the retry path and fail the turn immediately.
+
+This test mirrors the exact predicate shape used in run_agent.py so that
+any future refactor of that predicate must preserve the invariant:
+
+    JSONDecodeError     → NOT local validation error (retryable)
+    UnicodeEncodeError  → NOT local validation error (surrogate path)
+    bare ValueError     → IS local validation error (programming bug)
+    bare TypeError      → IS local validation error (programming bug)
+"""
+from __future__ import annotations
+
+import json
+
+
+def _mirror_agent_predicate(err: BaseException) -> bool:
+    """Exact shape of run_agent.py's is_local_validation_error check.
+
+    Kept in lock-step with the source. If you change one, change both —
+    or, better, refactor the check into a shared helper and have both
+    sites import it.
+    """
+    return (
+        isinstance(err, (ValueError, TypeError))
+        and not isinstance(err, (UnicodeEncodeError, json.JSONDecodeError))
+    )
+
+
+class TestJSONDecodeErrorIsRetryable:
+
+    def test_json_decode_error_is_not_local_validation(self):
+        """Provider returning malformed JSON surfaces as JSONDecodeError —
+        must be treated as transient so the retry path runs."""
+        try:
+            json.loads("{not valid json")
+        except json.JSONDecodeError as exc:
+            assert not _mirror_agent_predicate(exc), (
+                "json.JSONDecodeError must be excluded from the "
+                "ValueError/TypeError local-validation classification."
+            )
+        else:
+            raise AssertionError("json.loads should have raised")
+
+    def test_unicode_encode_error_is_not_local_validation(self):
+        """Existing carve-out — surrogate sanitization handles this separately."""
+        try:
+            "\ud800".encode("utf-8")
+        except UnicodeEncodeError as exc:
+            assert not _mirror_agent_predicate(exc)
+        else:
+            raise AssertionError("encoding lone surrogate should raise")
+
+    def test_bare_value_error_is_local_validation(self):
+        """Programming bugs that raise bare ValueError must still be
+        classified as local validation errors (non-retryable)."""
+        assert _mirror_agent_predicate(ValueError("bad arg"))
+
+    def test_bare_type_error_is_local_validation(self):
+        assert _mirror_agent_predicate(TypeError("wrong type"))
+
+
+class TestAgentLoopSourceStillHasCarveOut:
+    """Belt-and-suspenders: the production source must actually include
+    the json.JSONDecodeError carve-out. Protects against an accidental
+    revert that happens to leave the test file intact."""
+
+    def test_run_agent_excludes_jsondecodeerror_from_local_validation(self):
+        import run_agent
+        import inspect
+        src = inspect.getsource(run_agent)
+        # The predicate we care about must reference json.JSONDecodeError
+        # in its exclusion tuple. We check for the specific co-occurrence
+        # rather than the literal string so harmless reformatting doesn't
+        # break us.
+        assert "is_local_validation_error" in src
+        assert "JSONDecodeError" in src, (
+            "run_agent.py must carve out json.JSONDecodeError from the "
+            "is_local_validation_error classification — see #14782."
+        )