fix(remote-signer): validate retry configuration

[orkunsahin]

Summary

• Validate the nested RetryConfig from RemoteSigningConfig::validate().
• Preserve the existing retry validation error with additional configuration context.
• Add regression tests for invalid backoff ordering and invalid backoff multiplier.

Fixes #353.

Context

RetryConfig already defines validation rules for its backoff settings, but RemoteSigningConfig::validate() did not call the nested validator.

As a result, retry configurations rejected by RetryConfig::validate() could still pass the top-level configuration validation used by RemoteSignerClient.

This change delegates to the existing nested validator and reports the failure as an invalid retry configuration.

Testing

• cargo test -p arc-remote-signer
• cargo fmt --check
• cargo clippy -p arc-remote-signer --all-targets --all-features -- -D warnings
• git diff --check

Focused regression tests:

• remote_config_rejects_invalid_retry_backoff_order
• remote_config_rejects_invalid_retry_multiplier