Fix raw PCM input being decoded with the source codec (#6137)

# What does this implement/fix?

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`get_ffmpeg_args` declared the PCM decoder for a raw input container,
then unconditionally appended a second `-acodec` for
`input_format.codec_type`. ffmpeg applies the last `-acodec` it is
given, so whenever a caller passed a PCM `content_type` alongside a
populated `codec_type` the source codec's decoder was pointed at raw
samples.

Background audio analysis hits this on every track: it builds its PCM
format with `dataclasses.replace()` on the track's AudioFormat, which
swaps `content_type` but carries `codec_type` over from the encoded
source. The loudness provider's ebur128 process was therefore started as
"-f s16le -acodec pcm_s16le -acodec flac -i -", failing every packet
with "Invalid data found when processing input" until the 50-error guard
aborted the stream. The source files were never at fault.

Make the `codec_type` branch an `elif` so it only applies to encoded
input, where the container does not already imply the decoder.

Reproduced with a FLAC supplied in the bug report. The file decodes
cleanly; the failure was reproduced by replaying its decoded PCM through
the arg order the pre-fix code generates.

**Related issue (if applicable):**

- related issue https://github.com/music-assistant/support/issues/6267

## 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.

Co-authored-by: Claude <[email protected]>