Affected: pydantic httpx2

HTTPX2 is a next generation HTTP client for Python. Prior to 2.11.0, Request._prepare() in src/httpx2/httpx2/_models.py can add a body-derived Content-Length header to a request that already contains a caller-supplied Transfer-Encoding header because its setdefault() processing checks each default header independently rather than treating the two framing headers as mutually exclusive. Fixed-size byte, JSON, form, and known-length multipart bodies can therefore be serialized over HTTP/1.1 with both headers, allowing request smuggling or connection desynchronization when downstream intermediaries disagree about which framing header takes precedence. This issue is fixed in version 2.11.0.

Affected: PyPI: httpx2

### Summary

HTTPX2 can automatically add a `Content-Length` header to a request that already contains a caller-supplied `Transfer-Encoding` header. The resulting HTTP/1.1 request contains both framing headers, which can create an ambiguous message boundary and enable request smuggling or connection desynchronization when processed by intermediaries that disagree about which header takes precedence.

### Details

When a request body has a known size, HTTPX2's content encoder returns a default `Content-Length`. `Request._prepare()` applies each default header with `setdefault()`, which only checks whether that same header is already present. It does not check whether the mutually exclusive `Transfer-Encoding` header is present.

For example:

```python import httpx2

request = httpx2.Request( "POST", "http://example.com/", headers={"Transfer-Encoding": "chunked"}, content=b"test 123", )

print(request.headers) ```

The request contains both:

```text Transfer-Encoding: chunked Content-Length: 8 ```

On an HTTP/1.1 connection, the body is serialized using chunked transfer coding while both headers are sent on the wire. This violates HTTP message-framing requirements. Fixed-size byte, JSON, form, and known-length multipart bodies can reach the affected path.

Streaming bodies with an explicit `Content-Length` are not affected in current HTTPX2 releases because the automatically generated `Transfer-Encoding` is already suppressed in that direction.

### Impact

A…

Affected: PyPI: httpx2

### Summary

HTTPX2 can automatically add a `Content-Length` header to a request that already contains a caller-supplied `Transfer-Encoding` header. The resulting HTTP/1.1 request contains both framing headers, which can create an ambiguous message boundary and enable request smuggling or connection desynchronization when processed by intermediaries that disagree about which header takes precedence.

### Details

When a request body has a known size, HTTPX2's content encoder returns a default `Content-Length`. `Request._prepare()` applies each default header with `setdefault()`, which only checks whether that same header is already present. It does not check whether the mutually exclusive `Transfer-Encoding` header is present.

For example:

```python import httpx2

request = httpx2.Request( "POST", "http://example.com/", headers={"Transfer-Encoding": "chunked"}, content=b"test 123", )

print(request.headers) ```

The request contains both:

```text Transfer-Encoding: chunked Content-Length: 8 ```

On an HTTP/1.1 connection, the body is serialized using chunked transfer coding while both headers are sent on the wire. This violates HTTP message-framing requirements. Fixed-size byte, JSON, form, and known-length multipart bodies can reach the affected path.

Streaming bodies with an explicit `Content-Length` are not affected in current HTTPX2 releases because the automatically generated `Transfer-Encoding` is already suppressed in that direction.

### Impact

A…