Fix BteqOperator declaring template_fields as a bare string

[Eason09053360]

BteqOperator declared template_fields = "sql". A bare string is a sequence of
characters, so this declares the fields "s", "q" and "l" rather than "sql".

Airflow papers over this in BaseOperator.__init__, which wraps a string
template_fields in a list, but it emits a UserWarning every time a task is
built, and any code reading the class attribute directly — the sphinx
template-fields doc role, dry_run(), _set_xcomargs_dependencies() — still
sees the wrong value.

Declaring it as Sequence[str] = ("sql",) matches every other operator in the
codebase and drops the warning.

The existing test_template_fields asserted the incorrect value, so it is updated
to the corrected one (and a stray print removed).

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 5)

Generated-by: Claude Code (Opus 5) following the guidelines

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[potiuk]

Good catch. template_fields = "sql" declared three fields — "s", "q" and "l" — since a string is a sequence of characters.

The base operator wraps it at runtime so rendering still worked, but it warned on every task instantiation with "Please update {task_type} accordingly", and anything reading the class attribute before instantiation (the sphinx template-fields doc role, dry_run(), _set_xcomargs_dependencies()) saw the wrong value.

Nice that you also fixed the test — it was asserting == "sql", so it was pinning the bug in place.

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