docs(agents): Prefer Google Truth for new unit test assertions (#5735)

Steer AI coding agents toward Google Truth's fluent assertThat API for
assertions in new unit tests, while keeping kotlin.test for test
structure. Truth is already a dependency in the sentry module; note that
other modules need testImplementation(libs.google.truth) added before
use.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>