fix(OpenAIClient/PluginsClient): allow non-v1 reverse proxy, handle "v1/completions" reverse proxy

[danny-avila]

fixes #1026
fixes #1027

Aside from handling the niche case of a reverse proxy pointing to v1/completions instead of v1/chat/completions, you can also force a prompt payload instead of messages (useful for some localai models)

# (Advanced) Sometimes when using Local LLM APIs, you may need to force the API
# to be called with a `prompt` payload instead of a `messages` payload; to mimic the
# a `/v1/completions` request instead of `/v1/chat/completions`
# This may be the case for LocalAI with some models. To do so, uncomment the following:
# OPENAI_FORCE_PROMPT=true

Change Type

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation update

Testing

ci(OpenAIClient): add tests for OPENROUTER_API_KEY, FORCE_PROMPT, and reverseProxyUrl handling in setOptions

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• I have made pertinent documentation changes
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective or that my feature works
• Local unit tests pass with my changes
• Any changes dependent on mine have been merged and published in downstream modules.