GHSA-28jh-g32x-v9v4

### Summary

`Tesla.Multipart.part_headers_for_disposition/1` interpolates `Content-Disposition` parameter values (field name, filename, and other opts) verbatim into the part header line without encoding or escaping any special characters. An attacker who controls a filename, field name, or other disposition parameter can use unescaped double-quotes to inject extra disposition key-value pairs, or CRLF sequences to inject additional part headers or prepend bytes to the part body.

### Details

`part_headers_for_disposition/1` in `lib/tesla/multipart.ex` formats each disposition parameter as `k="v"` with no sanitization. Values flow in from `add_field/4` (the `name` argument), `add_file/3` and `add_file_content/4` (the `filename` argument and any disposition opts). A `"` in the value closes the quoted parameter early, allowing extra `; key="value"` pairs to be appended. A `\r\n` ends the `Content-Disposition` header line entirely, with subsequent bytes interpreted as additional part headers (e.g. a forged `Content-Type`); a second `\r\n` ends the whole part header block and prepends attacker bytes to the part body.

The default-filename path in `add_file/3` derives the name via `Path.basename/1`, which does not strip CR or LF, so any code that forwards a partially attacker-controlled file path is equally vulnerable.

### PoC

1. Call `Tesla.Multipart.add_file_content/4` with a filename containing `\r\nX-Injected: evil`. 2. POST the multipart body to any upstream via any Tesla adapter. 3. The upstream receives `X-Injected: evil` as a standalone header line on the affected part.

### Impact

Low severity (CVSS v4.0: 2.1). Any application using `tesla` 0.8.0 through 1.18.2 that passes untrusted input into `add_field/4`, `add_file/3`, or `add_file_content/4` disposition parameters is affected. Consequences range from forging part-level headers to body prepending against lenient multipart parsers. Fixed in tesla 1.18.3.

### Workarounds

Validate disposition parameter values before passing them to the multipart API, rejecting any value that contains `\r`, `\n`, or `"`.

### Resources

* Introduction commit: https://github.com/elixir-tesla/tesla/commit/6ebfdb9abe9c6f119408045b933d82462decd351 * Patch commit: https://github.com/elixir-tesla/tesla/commit/bb1a2c3da2775924d96e3db8e315dcc4d5d2246e