Python: Fix missing methods on the Content class in durable tasks (#4738)

* Fix Content serialization in DurableAgentStateUnknownContent (#4719)

DurableAgentStateUnknownContent.from_unknown_content() stored raw Content
objects without converting them to dicts, causing json.dumps to fail in
Azure Durable Functions' entity state serialization. This affected content
types not explicitly handled (e.g., mcp_server_tool_call/result).

The fix converts Content objects to dicts via to_dict() when storing in
DurableAgentStateUnknownContent, and restores them via Content.from_dict()
in to_ai_content().

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

* Add to_json and from_json methods to Content class (#4719)

Add to_json() and from_json() methods to the Content class to match the
serialization interface provided by SerializationMixin on other model classes.
Also fix pre-existing pyright type errors in durabletask's
DurableAgentStateUnknownContent.to_ai_content().

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

* Address PR review: add type guard, remove to_json, add fallback, and tests

- Remove Content.to_json() per reviewer request (comment 3)
- Add type guard in Content.from_json() for non-dict JSON (comments 1, 4)
- Wrap json.JSONDecodeError as ValueError for consistent exception contract
- Add try/except fallback in to_ai_content() for invalid Content dicts (comment 5)
- Add test_content_to_dict_exclude_none and test_content_to_dict_exclude_fields (comment 2)
- Add test_unknown_content_to_ai_content_fallback_on_invalid_type_dict (comment 5)

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

* Apply pre-commit auto-fixes

* Address review feedback for #4719: review comment fixes

* Remove Content.from_json, move logic to consuming code (#4719)

Remove the from_json convenience method from Content class per review
feedback. This is the same trivial json.loads + from_dict wrapper as
to_json which was already removed. Consumers should call json.loads
and Content.from_dict directly.

Update tests to use Content.from_dict(json.loads(...)) pattern and
remove from_json-specific error handling tests (those errors are
already covered by json.loads and Content.from_dict).

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

---------

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

@@ -1,6 +1,7 @@
 # Copyright (c) Microsoft. All rights reserved.
 
 import base64
+import json
 from collections.abc import AsyncIterable, Sequence
 from dataclasses import dataclass
 from datetime import datetime, timezone
@@ -1710,6 +1711,47 @@ def test_content_roundtrip_preserves_compaction_annotation_dict() -> None:
     assert annotation[GROUP_TOKEN_COUNT_KEY] is None
 
 
+def test_content_from_dict_via_json() -> None:
+    """Test Content.from_dict with data parsed from a JSON string."""
+    data = json.loads(json.dumps({"type": "text", "text": "Hello world"}))
+    content = Content.from_dict(data)
+    assert content.type == "text"
+    assert content.text == "Hello world"
+
+
+def test_content_from_dict_roundtrip_via_json() -> None:
+    """Test Content.from_dict roundtrip via to_dict and json.dumps."""
+    original = Content.from_function_call(call_id="call1", name="my_func", arguments={"key": "value"})
+    data = json.loads(json.dumps(original.to_dict()))
+    restored = Content.from_dict(data)
+    assert restored.type == "function_call"
+    assert restored.call_id == "call1"
+    assert restored.name == "my_func"
+    assert restored.arguments == {"key": "value"}
+
+
+def test_content_to_dict_exclude_none() -> None:
+    """Test Content.to_dict excludes None fields by default."""
+    content = Content.from_text("Hello")
+    d = content.to_dict()
+    parsed = json.loads(json.dumps(d))
+    assert "uri" not in parsed
+
+    d_with_none = content.to_dict(exclude_none=False)
+    parsed_with_none = json.loads(json.dumps(d_with_none))
+    assert "uri" in parsed_with_none
+    assert parsed_with_none["uri"] is None
+
+
+def test_content_to_dict_exclude_fields() -> None:
+    """Test Content.to_dict with explicit field exclusion."""
+    content = Content.from_text("Hello")
+    d = content.to_dict(exclude={"text"})
+    parsed = json.loads(json.dumps(d))
+    assert "text" not in parsed
+    assert parsed["type"] == "text"
+
+
 def test_chat_response_roundtrip_preserves_compaction_annotation_dict() -> None:
     response = ChatResponse(
         messages=[