fix(agentchat): remove dead typo attribute in CodeExecutorAgent

[nolanchic]

Why are these changes needed?

CodeExecutorAgent.__init__ assigns a misspelled attribute self._system_messaages (note the extra a in messaages) that is never read anywhere in the codebase:

self._system_messaages: List[SystemMessage] = []   # <- typo, never read
if system_message is None:
    self._system_messages = []                      # <- this is the one actually used
else:
    self._system_messages = [SystemMessage(content=system_message)]

The agent consistently uses the correctly-spelled self._system_messages (assigned on the very next lines, read at lines 525, 756, 757). A grep -rn "_system_messaages" across the whole repo confirms the misspelled form appears only on this single line — it is dead state created on every instantiation.

This removes the dead attribute. No behavior change.

Related issue number

N/A — trivial dead-code cleanup.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. (None — internal cleanup.)
• I've added tests (if relevant) corresponding to the changes introduced in this PR. (No new test — the change is a pure deletion of unreferenced state with no observable behavior. Existing CodeExecutorAgent tests still pass.)
• I've made sure all auto checks have passed. (Local verification below.)

Local verification

cd python
uv run pytest packages/autogen-agentchat/tests/test_code_executor_agent.py -q   # 26 passed
uv run ruff format --check packages/autogen-agentchat/src packages/autogen-agentchat/tests   # already formatted
uv run ruff check packages/autogen-agentchat/src packages/autogen-agentchat/tests   # all checks passed
uv run pyright packages/autogen-agentchat/src packages/autogen-agentchat/tests      # 0 errors
uv run mypy --config-file pyproject.toml packages/autogen-agentchat/src packages/autogen-agentchat/tests   # no issues