Add support for custom httpx client instances

Allow passing a pre-configured `httpx.Client` or `httpx.AsyncClient`
instance to reuse connections and custom configurations.

```python
import httpx
from ollama import Client

custom_httpx_client = httpx.Client(timeout=30.0)
client = Client(client=custom_httpx_client)

messages = [
    {
      'role': 'user',
      'content': 'Why is the sky blue?',
    },
]

for part in client.chat('gpt-oss:120b-cloud', messages=messages, stream=True):
    print(part.message.content, end='', flush=True)
```

Note: this is an updated minimal version of
https://github.com/ollama/ollama-python/pull/380 to support a custom
`httpx` client.

I took the approach of checking for the class instead of isinstance so
that nothing has to be a direct instance of an httpx.Client or
httpx.AsyncClient. It's up to the user to provide the right custom
client.

ollama/_client.py

@@ -1,4 +1,5 @@
 import contextlib
+import inspect
 import ipaddress
 import json
 import os
@@ -93,8 +94,18 @@ class BaseClient(contextlib.AbstractContextManager, contextlib.AbstractAsyncCont
     - `follow_redirects`: True
     - `timeout`: None
     `kwargs` are passed to the httpx client.
+
+    Args:
+      client: Either a httpx.Client/AsyncClient class, or an instance
     """
 
+    if not inspect.isclass(client):
+      assert follow_redirects is True, 'Cannot provide `follow_redirects` with custom client instance'
+      assert timeout is None, 'Cannot provide `timeout` with custom client instance'
+      assert not kwargs, 'Cannot provide additional kwargs with custom client instance'
+      self._client = client
+      return
+
     headers = {
       k.lower(): v
       for k, v in {
@@ -128,8 +139,8 @@ CONNECTION_ERROR_MESSAGE = 'Failed to connect to Ollama. Please check that Ollam
 
 
 class Client(BaseClient):
-  def __init__(self, host: Optional[str] = None, **kwargs) -> None:
-    super().__init__(httpx.Client, host, **kwargs)
+  def __init__(self, host: Optional[str] = None, *, client: Optional[httpx.Client] = None, **kwargs) -> None:
+    super().__init__(client or httpx.Client, host, **kwargs)
 
   def close(self):
     self._client.close()
@@ -721,8 +732,8 @@ class Client(BaseClient):
 
 
 class AsyncClient(BaseClient):
-  def __init__(self, host: Optional[str] = None, **kwargs) -> None:
-    super().__init__(httpx.AsyncClient, host, **kwargs)
+  def __init__(self, host: Optional[str] = None, *, client: Optional[httpx.AsyncClient] = None, **kwargs) -> None:
+    super().__init__(client or httpx.AsyncClient, host, **kwargs)
 
   async def close(self):
     await self._client.aclose()