fix(agent): classify TypeError('NoneType ... not iterable') as retryable provider shape error

Salvages the intent of #33136 (@Brixyy) onto current main. The original PR
was written against the pre-refactor monolithic run_agent.py and added a
top-level _is_nonretryable_local_validation_error() helper. Both target
functions have since been extracted to agent/conversation_loop.py:2869,
so the salvage applies the equivalent guard inline at that canonical
location rather than reintroducing the helper.

## Why

After #33042 made our own Codex consumer structurally immune to NoneType
crashes, third-party shims, mocked clients, and any future code path that
hasn't migrated could still surface TypeError: 'NoneType' object is not
iterable as a wire-shape mismatch. The agent loop's classifier currently
treats ALL TypeError as a local programming bug and aborts non-retryable
— users on stale Telegram/gateway turns saw bare "Non-retryable error
(HTTP None)" with no recovery.

This is a provider/SDK shape mismatch, not a local programming bug. The
retry/fallback path should run, not be short-circuited.

## What

agent/conversation_loop.py: extend is_local_validation_error to exclude
TypeErrors whose message matches the NoneType-not-iterable shape (case-
insensitive, both "NoneType" and "not iterable" must appear).

tests/run_agent/test_jsondecodeerror_retryable.py:
- update the mirror predicate to match the production check
- add TestNoneTypeNotIterableIsRetryable class with 3 tests (the basic
  shape, message variants, unrelated TypeErrors still abort)
- add TestAgentLoopSourceHasNoneTypeCarveOut to enforce the source-level
  invariant matches the test mirror

## Validation

tests/run_agent/test_jsondecodeerror_retryable.py +
tests/run_agent/test_31273_402_not_retried.py → 14/14 passing

Co-authored-by: Brixyy <subrtt@gmail.com>

agent/conversation_loop.py

@@ -2879,6 +2879,21 @@ def run_conversation(
                     # ssl.SSLError explicitly so the error classifier's
                     # retryable=True mapping takes effect instead.
                     and not isinstance(api_error, ssl.SSLError)
+                    # Provider/SDK "NoneType is not iterable" failures are
+                    # shape mismatches from upstream (e.g. chatgpt.com Codex
+                    # backend response.completed.output=null) — not local
+                    # programming bugs.  Even after #33042 made our own
+                    # consumer immune, third-party shims and mocked clients
+                    # can still surface this shape via TypeError.  Treat
+                    # them as retryable so the error classifier's normal
+                    # retry/fallback path runs instead of killing the turn
+                    # as non-retryable (which left Telegram users staring
+                    # at a bare "Non-retryable error" with no recovery).
+                    and not (
+                        isinstance(api_error, TypeError)
+                        and "nonetype" in str(api_error).lower()
+                        and "not iterable" in str(api_error).lower()
+                    )
                 )
                 # ``FailoverReason.billing`` (HTTP 402) is NOT in this
                 # exclusion set.  By the time we reach this block: