chore: fix remaining implicit Optional type hints, enforce with RUF013

[thomasht86]

Follow-up to #1341, which fixed implicit Optional hints across VespaSync/VespaAsync plus Vespa.query and Vespa.feed_data_point, but left four methods on the top-level Vespa class declaring namespace/groupname as str while defaulting them to None.

Those four are the most user-facing entry points in the library, and vespa/py.typed ships these signatures to downstream type checkers — so correct, documented usage produced errors in users' editors:

ns: Optional[str] = None
app.get_data(schema="s", data_id="1", namespace=ns)
# error: Argument "namespace" has incompatible type "str | None"; expected "str"

Why these annotations were wrong

None was always the intended value:

1. The docstrings already say so — e.g. namespace (str, optional): ... If no namespace is provided, the schema is used.
2. get_document_v1_path explicitly falls back to the schema name when namespace is falsy.
3. All four methods forwarded the argument verbatim into the VespaSync method that chore: fix optional type hints #1341 had already annotated Optional[str] — the wrappers were stricter than what they delegated to.

Scope

File	Change
vespa/application.py	7 params on Vespa.delete_data, delete_all_docs, get_data, update_data
vespa/configuration/vt.py	VT.__init__(attrs) — same bug class; body does attrs or {}
pyproject.toml	enable Ruff RUF013
tests/integration/ × 2	two helper signatures the new rule flagged

The last three are bundled deliberately: enabling RUF013 is what stops this from drifting back, and the rule fails on vt.py and the two test helpers, so they have to be fixed in the same commit or CI breaks on the next contributor's push. Ruff is already wired into pre-commit, so this adds no dependency and no new CI job.

Uses Optional[...] rather than PEP 604 str | None for consistency with #1341 and the surrounding code (351 vs 10 occurrences in vespa/). A full PEP 604 migration is viable — requires-python is >=3.10 — but it's a ~1,200-line diff across 14 files and belongs in its own change.

Verification

• pyright (the engine behind Pylance/VS Code) and mypy: both 0 errors on the reproducer above, which produced 4 errors each before.
• 668 unit tests pass; 51 doctests pass; integration tests collect cleanly.
• Internal mypy errors in the touched modules drop 72 → 64, with none added.
• ruff check . clean with RUF013 active; pre-commit hooks pass.

No runtime behaviour changes — annotations are not evaluated for defaults, and every affected parameter already accepted None.