Guard code_mode example DAG on SQLToolset import

[zachliu]

The example_agent_operator_code_mode DAG in providers/common/ai fails to parse when the optional [sql] extra (sqlglot) is not installed. It references SQLToolset at the module level without the if SQLToolset is not None: guard, so on a fresh environment SQLToolset is None and DAG parsing raises TypeError: 'NoneType' object is not callable.

This is a regression of the same class of bug fixed in #68497, which wrapped every SQL-dependent example DAG in this provider with the if X is not None: guard. The code-mode example was added afterward in #68407 and reintroduced the problem by omitting the guard.

This restores the established pattern: the DAG definition, its howto markers, and its registration call are all wrapped in if SQLToolset is not None:, so the DAG loads only when the optional dependency is present and no longer errors on a fresh dev environment.

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