fix: normalize vLLM Whisper API base URL

[ump45nose]

What this PR does / why we need it?

The vLLM Whisper provider appends /v1 unless the configured URL ends exactly with v1. A valid URL ending in /v1/ therefore becomes /v1//v1, while provider credential validation accepts the same configuration. This breaks transcription requests at runtime.

Summary of your change

• Strip trailing slashes before checking and appending the vLLM API version path.
• Add a focused regression test for an API URL ending in /v1/.

Validation:

• Focused runtime smoke with a stubbed OpenAI client: passed.
• Ruff fatal/static checks for the changed files: passed.
• Python syntax compilation and git diff --check: passed.

Please indicate you have done the following:

• Made sure focused tests are passing and test coverage is added where needed.
• Made sure the commit message follows the Conventional Commits specification.
• Considered the docs impact; no documentation change is needed for this normalization fix.