Document model_providers headers in configuration reference (#950)

* Document model_providers headers in configuration reference

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

* ci: retrigger workflow

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

* fix(llm_gateway): buffer non-streaming response body until end_of_stream

Wait for the full upstream body before JSON parsing to avoid truncated
responses on chunked replies. Retry currency_exchange demo tests on flake.

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

* fix(llm_gateway): read full non-streaming body when final chunk is empty

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

* fix(llm_gateway): read full non-streaming body with usize::MAX at end_of_stream

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

* fix(llm_gateway): use envoy body_size for response body replacement

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

* docs: explain model_providers headers in configuration reference

Revert unrelated llm_gateway and demo test runner changes.

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

* chore: drop unrelated changes, keep docs-only diff

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

---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>

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:

docs/source/resources/includes/plano_config_full_reference.yaml

@@ -47,6 +47,14 @@ model_providers:
     http_host: api.custom-provider.com
     access_key: $CUSTOM_API_KEY
 
+  # headers: optional map of extra HTTP headers sent on upstream requests (after auth).
+  # Use for provider-specific requirements such as User-Agent, org IDs, or account headers.
+  - model: moonshotai/kimi-for-coding
+    access_key: $MOONSHOTAI_API_KEY
+    base_url: https://api.kimi.com/coding/v1
+    headers:
+      User-Agent: "KimiCLI/1.3"
+
 # Model aliases - use friendly names instead of full provider model names
 model_aliases:
   fast-llm:

docs/source/resources/includes/plano_config_full_reference_rendered.yaml

@@ -88,6 +88,18 @@ listeners:
     port: 443
     protocol: https
     provider_interface: openai
+  - access_key: $MOONSHOTAI_API_KEY
+    base_url: https://api.kimi.com/coding/v1
+    base_url_path_prefix: /coding/v1
+    cluster_name: moonshotai_api.kimi.com
+    endpoint: api.kimi.com
+    headers:
+      User-Agent: KimiCLI/1.3
+    model: kimi-for-coding
+    name: moonshotai/kimi-for-coding
+    port: 443
+    protocol: https
+    provider_interface: moonshotai
   name: model_1
   output_filters:
   - input_guards
@@ -144,6 +156,18 @@ model_providers:
   port: 443
   protocol: https
   provider_interface: openai
+- access_key: $MOONSHOTAI_API_KEY
+  base_url: https://api.kimi.com/coding/v1
+  base_url_path_prefix: /coding/v1
+  cluster_name: moonshotai_api.kimi.com
+  endpoint: api.kimi.com
+  headers:
+    User-Agent: KimiCLI/1.3
+  model: kimi-for-coding
+  name: moonshotai/kimi-for-coding
+  port: 443
+  protocol: https
+  provider_interface: moonshotai
 - internal: true
   model: Plano-Orchestrator
   name: plano-orchestrator