Add manual trigger for CI workflow against beta PMS release

[JonnyWong16]

Description

Add a new manual trigger for tests against either public or beta Plex Media Server release channel. The test are skipped for unclaimed beta servers since a Plex Pass account is required.

Codecov uploads are also tagged with the PMS release channel.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable

[Copilot]

Pull request overview

This PR extends the CI workflow so maintainers can manually run tests against either the public or beta Plex Media Server channel, while propagating the selected channel into Docker image selection and Codecov tagging.

Changes:

• Add a workflow_dispatch choice input for selecting public vs beta PMS releases.
• Thread the selected release channel into the pytest matrix and PMS Docker tag selection.
• Include the release channel in Codecov flags for coverage uploads.

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

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