[Tracing] Emiting `TestcaseRejectionEvent` during triage

[vitaliset]

This PR introduces the emits of TestcaseRejectionEvent in the desired steps of the triage task life cycle as stated in the "Testcase rejection" section of "Lightweight Clusterfuzz Tracing" doc.

We are also adding assertions for verification of the emit.

The other remaining cases where TestcaseRejectionEvent must be emitted (namely fuzz task and corpus pruning) will be addressed in separate PRs.

b/394056013

[ViniciustCosta]

I will do a full review later, but just a heads up to avoid merging this one before actually testing it, since it touches critical part of the code (triage).

[vitaliset]

Superseded by #4846.