Fail Neo4jOperator argument mistakes at Dag parse time

[ColtenOuO]

Note

Blocked on #70505. The validate-operators-init hook on main still flags
is None provision checks in __init__, so CI will be red until that merges.
Kept as a draft until then — verified clean against the checker from #70505
(exit 0, no exemption entry needed).

Moves the two argument-provision checks of Neo4jOperator back to __init__, one of the reverts tracked in #70503.

Whether cypher or sql was passed is a property of how the Dag was written, not of the rendered value. execute() cannot answer it: with render_template_as_native_obj=True a supplied field can render to None, so the check there reports an argument the author did provide as missing. Checking at construction also turns a static authoring mistake into a Dag import error rather than a failure on every task instance.

Both provision checks move, not only cypher is None. After #70373 the operator keeps sql and cypher as separate fields, so Neo4jOperator(sql=...) is valid with cypher unset — the constructor check has to be cypher is None and sql is None. Once it is written that way, leaving the mutual-exclusivity check in execute() would split two checks that ask the same question across two phases.

The execute() guard is kept deliberately: Neo4jHook.run takes a str, and the constructor check does not narrow self.cypher to str (it is legitimately None when sql was used). A passed field can also render to None, which is exactly the case the guard covers. cast would hide that and assert is not allowed in production code.

Testing done

• providers/neo4j/tests/unit/neo4j/operators/test_neo4j.py: 6 passed. The two
  reverted tests assert construction-time failure again; a new test covers the
  execute() guard by rendering a supplied field to None.
• Checker from Allow argument-provision checks of templated fields in operator __init__ #70505 on the operator: exit 0 — no exemption entry needs re-adding.
• Checker on main: exit 1, as expected before Allow argument-provision checks of templated fields in operator __init__ #70505 merges.
• breeze run mypy on the operator: clean.

related: #70503, #70505

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Drafted-by: Claude Code (Opus 5); reviewed by @ColtenOuO before posting