GHSA-q7jx-v53g-848w

### Summary

`Tesla.Multipart.add_content_type_param/2` appends caller-supplied strings to the multipart `Content-Type` header with no validation. A param value containing `\r\n` splits the header line, allowing an attacker who controls any content-type parameter (charset, boundary parameter, etc.) to inject arbitrary headers into the outbound HTTP request.

### Details

`add_content_type_param/2` in `lib/tesla/multipart.ex` stores the supplied string directly in `multipart.content_type_params` without any CR/LF check. `headers/1` then joins all params with `"; "` and appends the result verbatim to the `Content-Type` header value. Because HTTP headers are delimited by `\r\n`, a param containing that sequence breaks out of the header field and introduces new header lines before the adapter writes the request to the socket.

The precondition is that untrusted input reaches `add_content_type_param/2`, which is the normal pattern for applications that accept user-supplied charset values, file type parameters, or any other content-type extension fields.

### PoC

1. Call `Tesla.Multipart.add_content_type_param/2` with a value containing `\r\nX-Injected: pwned`. 2. Pass the resulting `Multipart` struct as the request body via any Tesla adapter. 3. The raw request on the wire contains `X-Injected: pwned` as a standalone header line.

### Impact

Low severity (CVSS v4.0: 2.1). Any application using `tesla` 0.8.0 through 1.18.2 that passes untrusted input into `Tesla.Multipart.add_content_type_param/2` is affected. Consequences range from forging arbitrary outbound request headers to potential request smuggling against the upstream server. Fixed in tesla 1.18.3.

### Workarounds

Validate content-type parameter strings before passing them to `Tesla.Multipart.add_content_type_param/2`, rejecting any value that contains `\r` or `\n`.

### Reesources

* Introduction commit: https://github.com/elixir-tesla/tesla/commit/6ebfdb9abe9c6f119408045b933d82462decd351 * Patch commit: https://github.com/elixir-tesla/tesla/commit/23601edac5d22ba9407b427967b5bdbda201aec2