test: add regression test for mixed-unit offload false positive

Add test_multibyte_under_char_and_byte_limits_stays_inline which
exercises the specific #5561 regression: max_length=10000 with 3K
emoji chars (12K bytes).  Under the old mixed-unit min(), the byte
count exceeded the character-based threshold, triggering a false
offload.  With the fix, both limits are under their respective
thresholds and the text stays inline.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: caohy1988

tests/unittests/plugins/test_bigquery_agent_analytics_plugin.py

@@ -7498,3 +7498,34 @@ async def test_no_offloader_falls_back_to_truncate(self):
     assert is_truncated
     assert parts[0]["storage_mode"] == "INLINE"
     assert "TRUNCATED" in parts[0]["text"]
+
+  @pytest.mark.asyncio
+  async def test_multibyte_under_char_and_byte_limits_stays_inline(self):
+    """Multi-byte text under both char limit and byte limit stays inline.
+
+    This is the specific regression case from #5561: with the old
+    mixed-unit min(), max_length=10000 became the offload_threshold,
+    and byte_len (12K) > 10000 triggered a false offload even though
+    char_len (3K) < max_length and byte_len (12K) < inline_text_limit
+    (32KB).
+    """
+    mock_offloader = mock.AsyncMock()
+    parser = bigquery_agent_analytics_plugin.HybridContentParser(
+        offloader=mock_offloader,
+        trace_id="t",
+        span_id="s",
+        max_length=10000,
+    )
+
+    # 3K emoji chars → ~12K bytes
+    text = "\U0001f600" * 3000
+    assert len(text) < 10000  # under char limit
+    assert len(text.encode("utf-8")) > 10000  # bytes > max_length
+    assert len(text.encode("utf-8")) < 32 * 1024  # under byte limit
+
+    content = types.Content(parts=[types.Part(text=text)])
+    _, parts, _ = await parser._parse_content_object(content)
+
+    # Should NOT offload: under both real limits
+    mock_offloader.upload_content.assert_not_called()
+    assert parts[0]["storage_mode"] == "INLINE"