fix: SSE stream parser crashes with NoneType on providers sending null choice/usage/tc entries (#2389)

* fix: SSE parser crashes with NoneType on MiniMax-M3 (and any provider sending null choice/usage/tc)

Three guards added in stream_llm:

1. choices[0] null check — MiniMax (and some other providers) send a
   choices entry as None. `_choices[0].get("delta")` raised
   AttributeError. Now checks `_choices[0] is not None` before calling
   .get().

2. usage null guard — j["usage"] can arrive as None (not a dict) on
   some providers. Added `or {}` so subsequent .get() calls don't crash.

3. tool_calls null entry skip — individual entries in the tool_calls
   array can be None. Added `if tc is None: continue` before
   tc.get("function").

All three match the `or {}` / null-guard pattern used elsewhere in the
same block. Safe for all OpenAI-compatible providers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: guard null choice in elif-choices SSE branch

The usage-chunk path already guarded _choices[0] is not None, but the
elif "choices" branch that processes content/tool-call deltas did not.
A chunk like {"choices": [null]} or {"choices": [null], "usage": null}
reaches j["choices"][0].get("delta") and crashes with:

    'NoneType' object has no attribute 'get'

Fix: extract choices[0] into _c0 and continue to the next chunk when
it is None, matching the guard already applied in the usage path.

Adds three focused regressions covering the paths the maintainer flagged:
- {"choices": [null]}
- {"choices": [null], "usage": null}
- tool_calls array containing a null entry alongside a valid call

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_llm_core_usage_finish_delta.py

@@ -101,3 +101,56 @@ def test_usage_on_empty_choices_chunk_still_captured(monkeypatch):
     ]
     usage = _usage_events(_drive(monkeypatch, lines))
     assert usage and usage[-1] == {"input_tokens": 4, "output_tokens": 2}
+
+
+def test_null_choice_chunk_does_not_crash(monkeypatch):
+    # Some providers emit {"choices": [null]} as a heartbeat/keepalive chunk.
+    # The parser must silently skip it rather than crashing on None.get("delta").
+    lines = [
+        'data: ' + json.dumps({"choices": [{"delta": {"content": "Hello"}}]}),
+        'data: ' + json.dumps({"choices": [None]}),
+        'data: [DONE]',
+    ]
+    result = _drive(monkeypatch, lines)
+    assert "Hello" in result
+
+
+def test_null_choice_with_null_usage_does_not_crash(monkeypatch):
+    # Chunk with both choices:[null] and usage:null — neither field should panic.
+    lines = [
+        'data: ' + json.dumps({"choices": [{"delta": {"content": "Hi"}}]}),
+        'data: ' + json.dumps({"choices": [None], "usage": None}),
+        'data: [DONE]',
+    ]
+    result = _drive(monkeypatch, lines)
+    assert "Hi" in result
+
+
+def test_null_tool_call_in_delta_is_skipped(monkeypatch):
+    # Some providers include null entries in the tool_calls array alongside
+    # valid calls. The null entry must be skipped; the valid call must survive.
+    lines = [
+        'data: ' + json.dumps({
+            "choices": [{
+                "delta": {
+                    "tool_calls": [
+                        None,
+                        {"index": 0, "function": {"name": "get_weather", "arguments": '{"city":'}},
+                    ]
+                }
+            }]
+        }),
+        'data: ' + json.dumps({
+            "choices": [{
+                "delta": {
+                    "tool_calls": [
+                        {"index": 0, "function": {"name": "", "arguments": '"London"}'}},
+                    ]
+                }
+            }]
+        }),
+        'data: [DONE]',
+    ]
+    result = _drive(monkeypatch, lines)
+    # The stream completes without error; the valid tool call was accumulated.
+    assert result is not None