Allow unlimited connections per-worker (#488)

## Summary

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If this PR requires additional context or rationale, explain why
the changes are necessary.
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By default each httpx client supports a maximum of 100 connections
([ref](https://www.python-httpx.org/advanced/resource-limits/)). We want
this uncapped as connection count is maintained by a semaphore.

## Test Plan

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- See #487

## Related Issues

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- Resolves #487 

---

- [x] "I certify that all code in this PR is my own, except as noted
below."

## Use of AI

- [ ] Includes AI-assisted code completion
- [ ] Includes code generated by an AI application
- [ ] Includes AI-generated tests (NOTE: AI written tests should have a
docstring that includes `## WRITTEN BY AI ##`)

Author: sjmonson

src/guidellm/backends/openai.py

@@ -134,6 +134,12 @@ async def process_startup(self):
             timeout=self.timeout,
             follow_redirects=self.follow_redirects,
             verify=self.verify,
+            # Allow unlimited connections
+            limits=httpx.Limits(
+                max_connections=None,
+                max_keepalive_connections=None,
+                keepalive_expiry=5.0,  # default
+            ),
         )
         self._in_process = True