fix: avoid nil session dereference in OpenAI local mode

[anxkhn]

## What

The OpenAI agent executor crashes on every request when the A2A server is run
in local mode.

`KAgentApp.build_local()` constructs the executor with `session_factory=None`
(`python/packages/kagent-openai/src/kagent/openai/_a2a.py`), so `execute()`
leaves `session` as `None`. `_stream_agent_events` then built the
`SessionContext` with `session.session_id` unconditionally:

```python
session_context = SessionContext(session_id=session.session_id)

That dereference raises AttributeError: 'NoneType' object has no attribute 'session_id' before the agent runs, so no local-mode request ever reaches the
model.

Fix

Fall back to the A2A context id when there is no session:

session_id = session.session_id if session else context.context_id
session_context = SessionContext(session_id=session_id)

context.context_id is already how execute() derives the session id
(getattr(context, "session_id", None) or context.context_id) and is the
identifier used throughout this executor, so the SessionContext stays
consistent and stable per A2A conversation.

Testing

Added python/packages/kagent-openai/tests/test_agent_executor.py, which builds
the executor with session_factory=None and drives execute():

• one test asserts the run reaches Runner.run_streamed (before the fix it
  fails with the AttributeError above),
• one test asserts the SessionContext id falls back to context.context_id.

Both fail on main and pass with the fix.

cd python
uv run pytest packages/kagent-openai/tests/test_agent_executor.py

[Copilot]

Pull request overview

Fixes a crash in the OpenAI A2A agent executor when running in local mode (where session_factory=None) by avoiding dereferencing session.session_id when session is None, and adds regression tests to ensure local-mode execution reaches the runner and uses a stable session identifier.

Changes:

• Update _stream_agent_events() to derive a SessionContext.session_id even when session is None.
• Add async tests covering execute() behavior when session_factory=None, including validating the SessionContext id fallback.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
python/packages/kagent-openai/src/kagent/openai/_agent_executor.py	Avoids None session dereference by computing a fallback session id for SessionContext.
python/packages/kagent-openai/tests/test_agent_executor.py	Adds regression tests for local-mode execution and SessionContext session id fallback behavior.

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[peterj]

I agree with this comment - check for the session_id on context first, and then fallback to context_id, so it's same as in execute