fix: validate spork signer before processing

[thepastaclaw]

Summary

• Make CSporkManager::ProcessSpork() self-validate before mutating spork state
• Replace the optional signer assert()/.value() path with guarded optional handling

Validation

• git diff --check
• Confirmed no remaining GetSignerKeyID().value() / direct assert pattern in the touched spork path
• src/test/test_dash --run_test=spork_tests could not be run in this worktree because no test binary is present

Context: follow-up for CodeRabbit feedback on dashpay#7303.

[thepastaclaw]

Closing this follow-up because CodeRabbit withdrew the signer-assert concern as an intentional post-validation contract. I’ll replace it with the requested mainnet P2P spork follow-up instead.