refactor(gateway): use model_dump(exclude_unset=True) in messages route (#926)

## Description

Replaces 9 manual `if request.X is not None` conditionals and 3 provider
credential conditionals in the messages route with
`request.model_dump(exclude_unset=True)` merged with `provider_kwargs` —
matching the pattern already used in `chat.py`.

Net: **-32 lines, +3 lines**.

## PR Type

- 💅 Refactor

## Relevant issues

Fixes #915

## Checklist

- [x] I understand the code I am submitting.
- [x] I have added unit tests that prove my fix/feature works
- [x] I have run this code locally and verified it fixes the issue.
- [x] New and existing tests pass locally
- [x] Documentation was updated where necessary
- [x] I have read and followed the [contribution
guidelines](https://github.com/mozilla-ai/any-llm/blob/main/CONTRIBUTING.md)
- **AI Usage:**
    - [ ] No AI was used.
    - [ ] AI was used for drafting/refactoring.
    - [x] This is fully AI-generated.

## AI Usage Information

- AI Model used: Claude Opus 4.6
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: njbrake

src/any_llm/gateway/routes/messages.py

@@ -104,38 +104,9 @@ async def create_message(
 
     provider_kwargs = get_provider_kwargs(config, provider)
 
-    call_kwargs: dict[str, Any] = {
-        "model": f"{provider.value}:{model}",
-        "messages": request.messages,
-        "max_tokens": request.max_tokens,
-    }
-    if request.system is not None:
-        call_kwargs["system"] = request.system
-    if request.temperature is not None:
-        call_kwargs["temperature"] = request.temperature
-    if request.top_p is not None:
-        call_kwargs["top_p"] = request.top_p
-    if request.top_k is not None:
-        call_kwargs["top_k"] = request.top_k
-    if request.stop_sequences is not None:
-        call_kwargs["stop_sequences"] = request.stop_sequences
-    if request.tools is not None:
-        call_kwargs["tools"] = request.tools
-    if request.tool_choice is not None:
-        call_kwargs["tool_choice"] = request.tool_choice
-    if request.metadata is not None:
-        call_kwargs["metadata"] = request.metadata
-    if request.thinking is not None:
-        call_kwargs["thinking"] = request.thinking
-
-    # Pass provider credentials
-    if "api_key" in provider_kwargs:
-        call_kwargs["api_key"] = provider_kwargs["api_key"]
-    if "api_base" in provider_kwargs:
-        call_kwargs["api_base"] = provider_kwargs["api_base"]
-    client_args = provider_kwargs.get("client_args")
-    if client_args:
-        call_kwargs["client_args"] = client_args
+    # Request fields take precedence over provider config defaults
+    request_fields = request.model_dump(exclude_unset=True)
+    call_kwargs: dict[str, Any] = {**provider_kwargs, **request_fields}
 
     try:
         if request.stream: