Fix inconsistent base_url behavior in module client and OpenAIClient

Fixes openai#1373

Update `base_url` behavior to enforce trailing slash in module client

* Modify `src/openai/__init__.py` to enforce a trailing slash for `base_url` in the `_ModuleClient` class.
* Add a helper method `_enforce_trailing_slash` to the `_ModuleClient` class.
* Update tests in `tests/test_module_client.py` to verify the consistent behavior of `base_url` in both module client and `OpenAIClient`.

---

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Author: SmartManoj

src/openai/__init__.py

@@ -182,7 +182,7 @@ def project(self, value: str | None) -> None:  # type: ignore
     @override
     def base_url(self) -> _httpx.URL:
         if base_url is not None:
-            return _httpx.URL(base_url)
+            return self._enforce_trailing_slash(_httpx.URL(base_url))
 
         return super().base_url
 
@@ -245,6 +245,11 @@ def _client(self, value: _httpx.Client) -> None:  # type: ignore
 
         http_client = value
 
+    def _enforce_trailing_slash(self, url: _httpx.URL) -> _httpx.URL:
+        if url.raw_path.endswith(b"/"):
+            return url
+        return url.copy_with(raw_path=url.raw_path + b"/")
+
 
 class _AzureModuleClient(_ModuleClient, AzureOpenAI):  # type: ignore
     ...

tests/test_module_client.py

@@ -41,8 +41,8 @@ def test_base_url_option() -> None:
 
     openai.base_url = "http://foo.com"
 
-    assert openai.base_url == URL("http://foo.com")
-    assert openai.completions._client.base_url == URL("http://foo.com")
+    assert openai.base_url == URL("http://foo.com/")
+    assert openai.completions._client.base_url == URL("http://foo.com/")
 
 
 def test_timeout_option() -> None: