Python: duplicate User-Agent headers when caller passes lowercase user-agent

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### Motivation & Context

callers that pass a lowercase `user-agent` in default_headers (reachable through OpenAI client options) end up sending two User-Agent lines, because the prepend logic checks `"User-Agent" in headers` with exact casing and then adds its own.

### Reproduction

```python
prepend_agent_framework_to_user_agent({"user-agent": "my-app/1.0"})
# -> both "User-Agent: agent-framework/..." and the original lowercase one survive
# httpx sends duplicate header lines
```

### Expected behavior

header detection is case-insensitive (HTTP headers are case-insensitive); the framework value prepends to the caller's UA instead of duplicating it.

env: python main

Start at the prepend_agent_framework_to_user_agent function shown in the reproduction and trace how default_headers are checked before the framework value is added. Verify the lowercase user-agent case and confirm that the completed behavior preserves one header with the framework value prepended to the caller's value.

Written by the indexing model from the issue text.