Python: Include reasoning messages in MESSAGES_SNAPSHOT events (#4844)

* Include reasoning messages in MESSAGES_SNAPSHOT (#4843)

FlowState now tracks reasoning messages emitted during a run.
_emit_text_reasoning() persists reasoning (including encrypted_value)
into flow.reasoning_messages, and _build_messages_snapshot() appends
them to the final MESSAGES_SNAPSHOT event.

Changes:
- Add reasoning_messages field to FlowState
- Update _emit_text_reasoning() to accept optional flow parameter
- Include reasoning_messages in _build_messages_snapshot()
- Add 'reasoning' to ALLOWED_AGUI_ROLES so normalize_agui_role()
  preserves the role through snapshot round-trips
- Skip reasoning messages in agui_messages_to_agent_framework() since
  they are UI-only state and should not be forwarded to LLM providers
- Add regression tests for snapshot emission, encrypted value
  preservation, and multi-turn round-trip with reasoning

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

* Python: Include reasoning messages in MESSAGES_SNAPSHOT events

Fixes #4843

* Fix PR review feedback for reasoning persistence (#4843)

- Accumulate reasoning text per message_id (append deltas) instead of
  storing only the current chunk, matching flow.accumulated_text pattern
- Use camelCase encryptedValue in snapshot JSON to match AG-UI protocol
  conventions (toolCallId, encryptedValue)
- Normalize snake_case encrypted_value to encryptedValue in
  agui_messages_to_snapshot_format for input compatibility
- Update normalize_agui_role docstring to include reasoning role
- Add tests for incremental reasoning accumulation and key normalization

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

* Address review feedback for #4843: Python: agent-framework-ag-ui: include reasoning messages in MESSAGES_SNAPSHOT

---------

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

python/packages/ag-ui/tests/ag_ui/test_utils.py

@@ -450,6 +450,7 @@ def test_normalize_agui_role_valid():
     assert normalize_agui_role("assistant") == "assistant"
     assert normalize_agui_role("system") == "system"
     assert normalize_agui_role("tool") == "tool"
+    assert normalize_agui_role("reasoning") == "reasoning"
 
 
 def test_normalize_agui_role_invalid():