docs: explain model_providers headers in configuration reference

Revert unrelated llm_gateway and demo test runner changes.

Co-authored-by: Musa <musa@spherrrical.dev>

docs/source/resources/configuration_reference.rst

@@ -7,6 +7,29 @@ The following is a complete reference of the ``plano_config.yml`` that controls
 the Plano gateway. This where you enable capabilities like routing to upstream LLm providers, defining prompt_targets
 where prompts get routed to, apply guardrails, and enable critical agent observability features.
 
+Model provider headers
+----------------------
+
+Each entry under ``model_providers`` (or the legacy ``llm_providers`` alias) may include a ``headers`` map of extra
+HTTP headers that Plano adds to upstream LLM requests. Plano applies these headers after it sets authentication from
+``access_key`` or ``passthrough_auth``, so you can supply provider-specific metadata without replacing the configured
+credentials.
+
+- **Type:** map of strings (header name → value)
+- **Optional:** yes
+- **Common uses:** required ``User-Agent`` values, organization or account identifiers, or other headers some APIs expect
+
+.. code-block:: yaml
+
+    model_providers:
+      - model: moonshotai/kimi-for-coding
+        access_key: $MOONSHOTAI_API_KEY
+        base_url: https://api.kimi.com/coding/v1
+        headers:
+          User-Agent: "KimiCLI/1.3"
+
+The example below includes this and other provider options in context.
+
 .. literalinclude:: includes/plano_config_full_reference.yaml
     :language: yaml
     :linenos: