Fix PDA derivation ignoring per-call signer override in attest/createSchema

[Copilot]

attest() and createSchema() always used SdkConfig.currentSigner when deriving the attestation/schema PDA, ignoring any signer override passed at the call site. This caused incorrect or empty attestationId when a custom signer was provided.

Changes

• attest() — derive attester from (signer ?: SdkConfig.currentSigner) so the PDA matches the key that actually signed the transaction
• createSchema() — same fix applied for consistency

// Before
val attester = SdkConfig.currentSigner?.publicKey

// After
val attester = (signer ?: SdkConfig.currentSigner)?.publicKey

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