Fix broken nl-NL Alexa invocation phrase (#6164)

# What does this implement/fix?

The nl-NL command string used to trigger playback ("speel audio af op
music assistant") is not recognized by Alexa's NLU as a skill
invocation, causing Dutch users to hear "Ik heb niks gevonden met de
naam music assistant" ("I couldn't find anything named music assistant")
instead of the skill being triggered.

According to Amazon's official invocation grammar documentation for
nl-NL
(https://developer.amazon.com/en-US/docs/alexa/custom-skills/understanding-how-users-invoke-custom-skills.html),
the "Vraag" pattern only supports the connecting words naar/over/voor —
not "om" — and general phrasing built around "speel audio af op X"
doesn't map onto any of the supported nl-NL invocation templates at all,
so Alexa falls back to its generic device/skill search instead of
routing to the skill.

The bare launch phrase "speel music assistant" does match a supported
nl-NL pattern, and has been confirmed working both in the Alexa
developer console's simulator and in production on a real Echo device.

## Types of changes

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- [x] Bugfix (non-breaking change which fixes an issue) — `bugfix`
- [ ] New feature (non-breaking change which adds functionality) —
`new-feature`
- [ ] Enhancement to an existing feature — `enhancement`
- [ ] New music/player/metadata/plugin provider — `new-provider`
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected) — `breaking-change`
- [ ] Refactor (no behaviour change) — `refactor`
- [ ] Documentation only — `documentation`
- [ ] Maintenance / chore — `maintenance`
- [ ] CI / workflow change — `ci`
- [ ] Dependencies bump — `dependencies`

## Checklist

- [x] The code change is tested and works locally.
- [x] `pre-commit run --all-files` passes.
- [x] `pytest` passes, and tests have been added/updated under `tests/`
where applicable.
- [ ] For changes to shared models, the companion PR in
`music-assistant/models` is linked.
- [ ] For changes affecting the UI, the companion PR in
`music-assistant/frontend` is linked.
- [x] I have read and complied with the project's [AI
Policy](https://github.com/music-assistant/.github/blob/main/AI_POLICY.md)
for any AI-assisted contributions.
- [ ] I have [raised a PR against the documentation
repository](https://github.com/music-assistant/music-assistant.io/blob/main/CONTRIBUTING.md)
targeting the main or beta branch as appropriate.