Fix adapter error handling and remove redundant null checks

[Copilot]

Addressed review feedback on adapter error handling semantics and type safety violations.

Changes

• WebhookEngineAdapter: Changed signature validation failure outcome from Retry to Acknowledge - authentication failures are not transient errors
• WebhookEngineAdapter: Removed duplicate idempotency check block
• UiEngineAdapter: Removed redundant null check after ResolveEngine call - method signature guarantees non-null via where TContract : notnull
• ModuleEngineDiscoveryService: Fixed property access in error message - ModuleKey lives on descriptor, not manifest

// Before: infinite retry loop on auth failure
if (!signatureValidator.Validate(...))
    return new WebhookAdapterResponse(WebhookOutcomeType.Retry, "Signature validation failed");

// After: acknowledge and stop
if (!signatureValidator.Validate(...))
    return new WebhookAdapterResponse(WebhookOutcomeType.Acknowledge, "Signature validation failed");

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