[FR]: Re-raising signal for other handlers to catch

### Description

Since we register the signal handler ourselves first, Crashlytics will overwrite our signal handler when it registers the signal handler again. So I expect Crashlytics to re-raise the captured signal in [FIRCLSSignalHandler](https://github.com/firebase/firebase-ios-sdk/blob/95eb9dfe8bb2cca361522ec93436afebcb998f2a/Crashlytics/Crashlytics/Handlers/FIRCLSSignal.c#L327).

### API Proposal

_No response_

### Firebase Product(s)

Crashlytics

Start in Crashlytics/Crashlytics/Handlers/FIRCLSSignal.c at FIRCLSSignalHandler, and review how Crashlytics registers and captures signals. Verify the interaction with another handler such as Crashlytics' registration scenario, then ensure the captured signal is re-raised so other handlers can catch it. Done means the requested signal propagation behavior is covered by appropriate tests.

Written by the indexing model from the issue text.