Python: Fix _add_text_reasoning_content to preserve id during coalescing (#4862)

* Fix _add_text_reasoning_content dropping id during coalescing (#4852)

Preserve the id field (rs_* identifier) when coalescing text_reasoning
Content objects by passing id=self.id or other.id to the Content
constructor. This fixes the encrypted reasoning round-trip where the
missing id prevented _prepare_content_for_openai from including it in
the serialized reasoning item.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Python: Fix `_add_text_reasoning_content` to preserve `id` during coalescing

Fixes #4852

* Raise AdditionItemMismatch on conflicting text_reasoning ids (#4852)

Detect when both operands have different non-empty ids during
text_reasoning Content coalescing and raise AdditionItemMismatch
instead of silently keeping one. This prevents mis-associating
encrypted_content during round-trips.

Also adds tests for conflicting ids and the neither-has-id edge case.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address review feedback for #4852: Python: [Bug]: Content._add_text_reasoning_content drops id during coalescing, breaking encrypted reasoning round-trip

* test fix

---------

Co-authored-by: Copilot <copilot@github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/packages/core/tests/core/test_types.py

@@ -43,7 +43,7 @@ from agent_framework._types import (
     add_usage_details,
     validate_tool_mode,
 )
-from agent_framework.exceptions import ContentError
+from agent_framework.exceptions import AdditionItemMismatch, ContentError
 
 
 @fixture
@@ -1526,6 +1526,88 @@ def test_text_reasoning_content_iadd_coverage():
     assert t1.text == "Thinking 1 Thinking 2"
 
 
+def test_text_reasoning_content_add_preserves_id():
+    """Test that coalescing text_reasoning Content preserves the id field."""
+
+    t1 = Content.from_text_reasoning(id="rs_abc123", text="Thinking part 1")
+    t2 = Content.from_text_reasoning(id="rs_abc123", text=" part 2")
+
+    result = t1 + t2
+    assert result.text == "Thinking part 1 part 2"
+    assert result.id == "rs_abc123"
+
+
+def test_text_reasoning_content_add_id_fallback_to_other():
+    """Test that coalescing falls back to other's id when self has no id."""
+
+    t1 = Content.from_text_reasoning(text="Thinking part 1")
+    t2 = Content.from_text_reasoning(id="rs_abc123", text=" part 2")
+
+    result = t1 + t2
+    assert result.id == "rs_abc123"
+
+
+def test_text_reasoning_content_add_preserves_id_with_encrypted_content():
+    """Test that id and encrypted_content both survive coalescing for round-trip."""
+
+    t1 = Content.from_text_reasoning(
+        id="rs_abc123",
+        text="Thinking",
+        additional_properties={"encrypted_content": "enc_blob_data"},
+    )
+    t2 = Content.from_text_reasoning(id="rs_abc123", text=" more")
+
+    result = t1 + t2
+    assert result.text == "Thinking more"
+    assert result.id == "rs_abc123"
+    assert result.additional_properties.get("encrypted_content") == "enc_blob_data"
+
+
+def test_text_reasoning_content_add_conflicting_ids_raises():
+    """Test that coalescing text_reasoning Content with different ids raises AdditionItemMismatch."""
+
+    t1 = Content.from_text_reasoning(id="rs_abc123", text="Thinking part 1")
+    t2 = Content.from_text_reasoning(id="rs_xyz789", text=" part 2")
+
+    with pytest.raises(AdditionItemMismatch, match="different ids"):
+        t1 + t2
+
+
+def test_text_reasoning_content_add_neither_has_id():
+    """Test that coalescing text_reasoning Content when neither has an id results in None id."""
+
+    t1 = Content.from_text_reasoning(text="Thinking part 1")
+    t2 = Content.from_text_reasoning(text=" part 2")
+
+    result = t1 + t2
+    assert result.text == "Thinking part 1 part 2"
+    assert result.id is None
+
+
+def test_coalesce_text_reasoning_with_different_ids():
+    """Test that _coalesce_text_content keeps separate text_reasoning items when IDs differ.
+
+    Regression test: streaming responses can produce multiple text_reasoning
+    segments with distinct IDs. These must not be merged into one.
+    """
+    from agent_framework._types import _coalesce_text_content
+
+    contents = [
+        Content.from_text_reasoning(id="rs_aaa", text="Thinking A1"),
+        Content.from_text_reasoning(id="rs_aaa", text=" A2"),
+        Content.from_text_reasoning(id="rs_bbb", text="Thinking B1"),
+        Content.from_text_reasoning(id="rs_bbb", text=" B2"),
+    ]
+
+    _coalesce_text_content(contents, "text_reasoning")
+
+    assert len(contents) == 2
+    assert contents[0].id == "rs_aaa"
+    assert contents[0].text == "Thinking A1 A2"
+    assert contents[1].id == "rs_bbb"
+    assert contents[1].text == "Thinking B1 B2"
+
+
 def test_comprehensive_to_dict_exclude_options():
     """Test to_dict methods with various exclude options for better coverage."""