feat: check for existing request_class config

[cofin]

Description

Closes

Summary by Sourcery

Introduce concurrency control in the CI workflow and enhance the plugin to check for an existing request_class configuration before setting it.

New Features:

• Add concurrency control to GitHub Actions CI workflow to cancel in-progress runs for the same branch.

Enhancements:

• Modify the plugin to check for an existing request_class configuration before setting it to HTMXRequest.

CI:

• Introduce concurrency control in the CI workflow to manage simultaneous runs on the same branch.

[sourcery-ai]

Reviewer's Guide by Sourcery

This pull request introduces two main changes: it adds concurrency control to the CI workflow and modifies the HTMX plugin to check for an existing request_class configuration before setting it.

No diagrams generated as the changes look simple and do not need a visual representation.

File-Level Changes

Change	Details	Files
Add concurrency control to CI workflow	Introduce a concurrency group named 'test-${{ github.head_ref }}' Enable cancellation of in-progress workflows	.github/workflows/ci.yml
Modify HTMX plugin to check for existing request_class configuration	Add a conditional check before setting app_config.request_class Only set app_config.request_class to HTMXRequest if it's currently None	litestar_htmx/plugin.py

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[sourcery-ai]

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