Harden SimpleStringBuilder safety checks

Replace RTC_DCHECK with RTC_CHECK in SimpleStringBuilder to ensure
safety constraints are enforced in all build configurations, including
release builds. This change prevents undefined behavior or silent
truncation by crashing when a buffer overflow is detected, rather than
only performing these checks in debug modes.

The modifications include:
* Updating SimpleStringBuilder to use RTC_CHECK for consistency and
  bounds verification.
* Simplifying RtpExtension::ToString and VideoReceiveStream2 by
  migrating from SimpleStringBuilder to StringBuilder.
* Simplify+clarify the StringBuilder::Release implementation.
* Update unit tests to reflect that overflow now consistently results
  in a crash.

Bug: chromium:486536241
Change-Id: Ia067508e2b9409154ad4afdbe4c0971a1dd32e1e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/459441
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47251}