### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

Following an example at https://docs.litellm.ai/docs/integrations/websearch_interception I've set this in the config

``` search_tools: - search_tool_name: perplexity-search litellm_params: search_provider: perplexity api_key: os.environ/FOO_BAR ```

Note that the `api_key` is using a custom env var name.

Then I tried to use search and got following error

``` WebSearchInterception: Search 0 failed with error: litellm.APIConnectionError: APIConnectionError: PerplexityException - PERPLEXITYAI_API_KEY is not set. Set `PERPLEXITYAI_API_KEY` environment variable. ```

I would expect LLM to use env var name specified by me in the config, not forced hardcoded value.

### Steps to Reproduce

1. set `FOO_BAR` env var containing Perplexity API key 2. configure perplexity search tool 3. call search tool

### Relevant log output

```shell WebSearchInterception: Search 0 failed with error: litellm.APIConnectionError: APIConnectionError: PerplexityException - PERPLEXITYAI_API_KEY is not set. Set `PERPLEXITYAI_API_KEY` environmen…