Address code review feedback: optimize registry lookups, add input validation, improve diagnostics

[Copilot]

Addresses performance, validation, and documentation feedback from PR review comments.

Performance Optimization

Registry lookups refactored to avoid full snapshots:

• FindWebhookEngine and FindById now search lists directly under locks instead of calling GetEngines()
• FindById narrows to specific moduleKey before iteration, eliminating unnecessary scans
• Removed redundant moduleKey comparison after dictionary lookup

Input Validation

Adapters now validate parameters at entry:

• WebhookEngineAdapter: validates Provider and EventType are non-empty
• UiEngineAdapter: validates moduleKey and engineId are non-empty
• Both throw ArgumentException with clear messages on invalid input

Error Messages

Improved diagnostic clarity:

• ModuleRegistry: explains why descriptor moduleKey must match owning module and how to fix
• WebhookAdapter idempotency: specifies that key is required when window is configured

Documentation

Clarified reserved and ambiguous parameters:

• SupportsAsync: documented as reserved for future use (all engines are currently async)
• WebhookRequest.Attempt: documented as 1-based for callers despite 0 default
• RequiredServices: added engine-overview.md section explaining host validation expectations

Test Coverage

Added tests for error paths:

[Fact]
public async Task Webhook_adapter_rejects_invalid_signature()
{
    // ... invalid signature header ...
    var response = await adapter.DispatchAsync(request, CancellationToken.None);
    Assert.Equal(WebhookOutcomeType.Acknowledge, response.Outcome);
}

[Fact]
public async Task Ui_engine_exception_propagates_to_adapter()
{
    await Assert.ThrowsAsync<ArgumentException>(async () =>
        await adapter.ExecuteAsync<LoginCommand, LoginViewModel>(
            "fake-module", "ui.login", new LoginCommand(string.Empty, "pass"), ...));
}

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