Fix convert --format with never_convert_lossy_files

[GuyBloom]

Description

Fixes #5625

When convert.never_convert_lossy_files is enabled, beet convert was
ignoring the explicit --format option and just copying the lossy files without
transcoding them. For example:

• beet convert format:mp3 --format opus

would still produce MP3 files instead of OPUS.

Change:

• Threads information about whether --format was provided on the CLI down
  into the convert plugin.

• Allows an explicit --format to override never_convert_lossy_files for
  beet convert, so lossy files selected by the query are transcoded to the
  requested format.

• Keeps existing behavior for automatic conversion on import (no CLI
  override there).

• Adds a regression test in NeverConvertLossyFilesTest to cover this case.

• Documents the behavior in the convert plugin docs and adds a changelog entry.

• Documentation. (Clarified the --format override interaction in docs/plugins/convert.rst)

• Changelog. (Added entry to docs/changelog.rst)

• Tests. (New test added to test/plugins/test_convert.py to assert that --format forces transcoding even when never_convert_lossy_files is enabled.)

[sourcery-ai]

Hey there - I've reviewed your changes - here's some feedback:

• The new override_never_lossy flag is threaded through many calls—consider encapsulating CLI override state in the plugin instance or a context object to simplify these long method signatures.
• Rename override_never_lossy to something like force_transcode or force_convert for clearer intent when reading the should_transcode code path.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The new override_never_lossy flag is threaded through many calls—consider encapsulating CLI override state in the plugin instance or a context object to simplify these long method signatures.
- Rename override_never_lossy to something like force_transcode or force_convert for clearer intent when reading the should_transcode code path.

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[GuyBloom]

Hi! My PR is failing the "Get changed docs files" check, but it seems to be a workflow configuration issue rather than a problem with my code.
The error message indicates: "Failed to fetch pull request branch. Please ensure 'persist-credentials' is set to 'true' when checking out the repository."
Looking at the logs, the tj-actions/changed-files@v46 action can't fetch the PR branch history. This might need a fix in the workflow file to add persist-credentials: true to the checkout step.
Could someone with write access take a look? Happy to help test any fixes if needed!

[JOJ0]

Hmm the typical cli-way for overriding something that is configured is a --force option. Would you mind adding one and changing the logic? That way convert --help could also explain this feature most obvious without having to find out it's even existing by looking it up in the docs.

[GuyBloom]

Got it, thanks for the suggestion. I’ve added a --force option to beet convert which overrides no_convert and never_convert_lossy_files. The docs are updated and I've added a regression test for this issue