fix(passkeys): gate rpId/allowedOrigins validation on enabled flag

[vpomerleau]

Because

• buildPasskeyConfig validated rpId and allowedOrigins unconditionally, causing auth-server to crash at startup on any environment where passkeys is disabled but the required fields are left at their Convict defaults (empty string / empty array)

This pull request

• Adds @ValidateIf((o) => o.enabled) to rpId and allowedOrigins in PasskeyConfig so those constraints are skipped when the feature is off
• Wraps the buildPasskeyConfig call in key_server.js with try/catch so that misconfigured-but-enabled passkeys logs via log.error (structured mozlog) before exiting, instead of falling through to console.error
• Adds a test asserting that disabled passkeys with empty defaults no longer throws

Issue that this pull request solves

Closes: FXA-13378

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