feat: add http_headers support to LlmProvider for custom upstream headers (fixes #592)

Allows users to specify custom HTTP request headers per LLM provider in
plano_config.yaml, useful for providers requiring non-standard auth or
telemetry headers:

  model_providers:
    - model: provider/model
      access_key: $API_KEY
      http_headers:
        X-Custom-Header: "custom-value"
        X-API-Version: "v1"

crates/common/src/configuration.rs

@@ -445,6 +445,7 @@ pub struct LlmProvider {
     pub base_url_path_prefix: Option<String>,
     pub internal: Option<bool>,
     pub passthrough_auth: Option<bool>,
+    pub http_headers: Option<HashMap<String, String>>,
 }
 
 pub trait IntoModels {
@@ -488,6 +489,7 @@ impl Default for LlmProvider {
             base_url_path_prefix: None,
             internal: None,
             passthrough_auth: None,
+            http_headers: None,
         }
     }
 }

crates/common/src/llm_providers.rs

@@ -277,6 +277,7 @@ mod tests {
             internal: None,
             stream: None,
             passthrough_auth: None,
+            http_headers: None,
         }
     }