fix: route Perplexity OpenAI endpoints without /v1 (#854)

* fix: route Perplexity OpenAI paths without /v1

* add tests for Perplexity provider handling in LLM module

* refactor: use constant for Perplexity provider prefix in LLM module

* moving const to top of file

crates/llm_gateway/src/stream_context.rs

@@ -122,6 +122,7 @@ impl StreamContext {
                     .unwrap_or(&"".to_string()),
                 self.streaming_response,
                 self.llm_provider().base_url_path_prefix.as_deref(),
+                self.llm_provider().name.starts_with("perplexity/"),
             );
             if target_endpoint != request_path {
                 self.set_http_request_header(":path", Some(&target_endpoint));