feat(openai): add configurable base_url support with OPENAI_BASE_URL env var (#2919)

# What does this PR do?

- Add base_url field to OpenAIConfig with default
"https://api.openai.com/v1"
- Update sample_run_config to support OPENAI_BASE_URL environment
variable
- Modify get_base_url() to return configured base_url instead of
hardcoded value
- Add comprehensive test suite covering:
  - Default base URL behavior
  - Custom base URL from config
  - Environment variable override
  - Config precedence over environment variables
  - Client initialization with configured URL
  - Model availability checks using configured URL

This enables users to configure custom OpenAI-compatible API endpoints
via environment variables or configuration files.

Closes #2910 

## Test Plan

run unit tests

llama_stack/templates/starter/run.yaml

@@ -56,6 +56,7 @@ providers:
     provider_type: remote::openai
     config:
       api_key: ${env.OPENAI_API_KEY:=}
+      base_url: ${env.OPENAI_BASE_URL:=https://api.openai.com/v1}
   - provider_id: anthropic
     provider_type: remote::anthropic
     config: