Enable the integration tests for vllm

[rdimitrov]

The following PR enables the integration tests for vllm.

Details:

• Starts the CodeGate container in the host network (so everything can talk to each other without the need to specify explict ports), we don't have port conflicts anyway
• Starts the vllm container, runs the Qwen/Qwen2.5-Coder-0.5B-Instruct model and verifies if it is serving as expected
• Updates the vllm tests to use Qwen/Qwen2.5-Coder-0.5B-Instruct
• Updates the chat test to expect a simpler welcome message
• Updates the vllm provider making the Authorization header optional

Fixes: #816

Nit: For some reason I wasn't able to get vllm running on my Mac (issue).

What's left:

• Figure out why the FIM test is failing. Note that I tried with 1.5B and 3B models too. The 1.5B failed, whereas the 3B model took more than 1hr of CI to compute and I canceled it at the end, most certainly because we are running on CPU mode only in a low-performant environment).

Logs:

• From trying with Qwen/Qwen2.5-Coder-3B-Instruct - https://github.com/stacklok/codegate/actions/runs/13030195063/job/36347712608?pr=806
• From trying with Qwen/Qwen2.5-Coder-0.5B https://github.com/stacklok/codegate/actions/runs/13032337694/job/36354452847?pr=806

[jhrozek]

really nice work!

[lukehinds]

+1 awesome stuff