A flaw was found in Netty RtspDecoder. The `RtspMethods.valueOf()` function incorrectly strips trailing control bytes from method tokens in Real-Time Streaming Protocol (RTSP) requests. A remote attacker can exploit this by sending a specially crafted RTSP request, leading to method-token smuggling. This vulnerability allows an attacker to bypass method-based access controls and can also be used to launder malicious requests through Netty-based RTSP proxies, making them appear legitimate to backend systems.

### Summary `RtspMethods.valueOf()` silently strips trailing control bytes (any character with code point <= 0x20, the full range that `String.trim()` removes) before performing a cache lookup against its ten pre-populated method constants. A wire-delivered RTSP request whose method token ends with a trailing control byte — for example `PLAY\x00` or `PLAY\r`, immediately before the separating space — is decoded by `RtspDecoder` as a fully successful PLAY request, with `decoderResult().isSuccess() == true and request.method() == RtspMethods.PLAY` (same object reference as the cached singleton). The application layer cannot distinguish this from a clean `PLAY` request.

This is the same root cause as #16723 and #16971, in a sibling that those fixes did not reach. The fix for `HttpMethod` hardened `HttpMethod.valueOf()` directly, but `RtspMethods.valueOf()` has its own independent `checkNonEmptyAfterTrim()` call that runs before the cache lookup — meaning a trailing-control-byte token hits the cache before the hardened `HttpMethod` constructor ever sees it.

### Reproduction

Minimal wire-level reproduction

Send the following raw bytes to any Netty-based RTSP server using R

netty-4.1.138-1.1 on GA media

Affected: Ubuntu:Pro:14.04:LTS: netty, Ubuntu:Pro:16.04:LTS: netty, Ubuntu:Pro:18.04:LTS: netty, Ubuntu:Pro:20.04:LTS: netty, Ubuntu:Pro:22.04:LTS: netty, Ubuntu:Pro:24.04:LTS: netty, Ubuntu:Pro:26.04:LTS: netty

A flaw was found in Netty RtspDecoder. The `RtspMethods.valueOf()` function incorrectly strips trailing control bytes from method tokens in Real-Time Streaming Protocol (RTSP) requests. A remote attacker can exploit this by sending a specially crafted RTSP request, leading to method-token smuggling. This vulnerability allows an attacker to bypass method-based access controls and can also be used to launder malicious requests through Netty-based RTSP proxies, making them appear legitimate to backend systems.