fix(custom): pass custom provider extra body

Allow custom OpenAI-compatible providers declared under `custom_providers:`
to set provider-specific `extra_body` fields and have Hermes merge them into
chat-completions requests when the matching custom endpoint is active.

This is a manual per-provider override rather than a model-name heuristic.
OpenAI-compatible Gemma thinking support is real, but the on-wire payload
shape is backend-specific: some servers want top-level `enable_thinking`,
while vLLM Gemma and NIM-style endpoints expect `chat_template_kwargs`.
A per-provider override is safer than picking one assumed payload.

Example config:

```yaml
custom_providers:
  - name: gemma-local
    base_url: http://localhost:8080/v1
    model: google/gemma-4-31b-it
    extra_body:
      enable_thinking: true
      reasoning_effort: high
```

For vLLM Gemma or NIM-style endpoints, use the nested shape those servers
expect:

```yaml
extra_body:
  chat_template_kwargs:
    enable_thinking: true
```

Changes:

- `hermes_cli/config.py`: preserve `extra_body` in normalized
  `custom_providers:` entries and allow it in the validated field set.
- `hermes_cli/runtime_provider.py`: propagate custom-provider `extra_body`
  as `request_overrides.extra_body` for named custom runtime resolution,
  including credential-pool paths.
- `agent/agent_init.py`: at agent init, locate the matching custom-provider
  entry by `base_url` (+ optional model) and merge its `extra_body` into
  `AIAgent.request_overrides`, with caller-provided overrides winning on
  conflicting top-level keys.
- `plugins/model-providers/custom/__init__.py`: keep existing CustomProfile
  behavior (Ollama `num_ctx`, `think=False` when reasoning disabled);
  user-configured `extra_body` flows through `request_overrides`.
- `website/docs/integrations/providers.md`: document the explicit
  `extra_body` override and the vLLM/Gemma `chat_template_kwargs` variant.
- Tests cover config normalization, runtime propagation, model matching,
  trailing-slash equivalence, fallback when no `model` field is set, and
  caller-override merging precedence.

Verified end-to-end against `CustomProfile` via `ChatCompletionsTransport`:
configured `extra_body` reaches `kwargs.extra_body` on the wire request,
and coexists with profile-generated entries (Ollama `num_ctx`, `think=False`)
without clobber.

Salvaged from #29022 onto current `main`. Cosmetic typing edit in
`plugins/model-providers/custom/__init__.py` and a stale-base docs revert
in `providers.md` were dropped during cherry-pick.

Closes #29022

website/docs/integrations/providers.md

@@ -1228,6 +1228,26 @@ custom_providers:
     api_mode: anthropic_messages  # for Anthropic-compatible proxies
 ```
 
+Some OpenAI-compatible endpoints need provider-specific request body fields. Add an `extra_body` map to the matching custom provider and Hermes will merge it into each chat-completions request for that endpoint:
+
+```yaml
+custom_providers:
+  - name: gemma-local
+    base_url: http://localhost:8080/v1
+    model: google/gemma-4-31b-it
+    extra_body:
+      enable_thinking: true
+      reasoning_effort: high
+```
+
+Use the shape your server documents. For example, vLLM Gemma deployments and some NVIDIA NIM endpoints expect `enable_thinking` under `chat_template_kwargs` instead of as a top-level `extra_body` field:
+
+```yaml
+extra_body:
+  chat_template_kwargs:
+    enable_thinking: true
+```
+
 The `hermes model` → Custom Endpoint wizard now prompts for `api_mode` explicitly and persists your answer to `config.yaml`. URL-based auto-detection (e.g. `/anthropic` paths → `anthropic_messages`) still happens as a fallback when the field is left blank.
 
 Switch between them mid-session with the triple syntax: