Refactor signer name consistency enforcement in AppendOptions validation

[roger2hk]

This pull request defers the signer validation in WithCheckpointSigner to avoid os.Exit and context.Background.

Problem
The WithCheckpointSigner method was performing immediate validation on the provided signers. However, this method is a configuration setter and does not take a context.Context. Only context.Background() can be used to log errors when validation failed. Furthermore, it called os.Exit(1) directly upon failure, which is anti-idiomatic for library code as it prevents the calling application from handling the failure gracefully.

Solution
This change defers the validation of additional signers to the valid() method of AppendOptions. If validation fails, valid() returns an error instead of crashing the process. This approach does not cause any breaking changes.

Why better?

• Idiomatic Go Error Handling: Libraries should return errors rather than calling os.Exit. This allows the application using Tessera to decide how to handle invalid configurations.
• Eliminates dummy contexts: By moving the check to valid() (which is called by NewAppender in a context-aware environment), we no longer need to use context.Background() for logging errors.

[AlCutter]

Could you add a description which explains why this change is necessary/better?

[roger2hk]

Could you add a description which explains why this change is necessary/better?

Sure, I've just updated the pull request description.