This PR implements validation to prevent misconfigured VMR codeflow subscriptions that could lead to conflicts and operational issues, while also improving the API design by unifying subscription querying methods.

Problem

Currently, it's possible to create conflicting codeflow subscriptions that can cause problems:

1. Backflow conflicts: Multiple VMR→repository subscriptions targeting the same repository branch
2. Forward flow conflicts: Multiple repository→VMR subscriptions flowing into the same VMR branch and target directory

These misconfigurations can lead to race conditions, merge conflicts, and unexpected behavior in the dependency flow system.

Solution

Comprehensive Validation

Added validation with a shared service architecture:

• ICodeflowSubscriptionValidationService: Common interface in Maestro.Common
• CodeflowSubscriptionValidationService: Single implementation containing all validation logic
• Server-side validation: SubscriptionsController validates on Create() and UpdateSubscription()
• Client-side validation: DARC CLI validates in AddSubscriptionOperation and UpdateSubscriptionOperation
• Shared base class: SubscriptionOperationBase eliminates duplication between Add/Update operations

Validation Rules

Backflow subscriptions (VMR → repository, sourceDirectory set):

• Only one backflow subscription allowed per target repository + target branch combination

Forward flow subscriptions (repository → VMR, targetDirectory set):

• Only one forward flow subscription allowed per VMR repository + VMR branch + target directory combination

API Improvement

Replaced the specific GetCodeflowSubscriptionsAsync method with a more generic GetSubscriptionsAsync that accepts additional optional parameters:

Task<IEnumerable<Subscription>> GetSubscriptionsAsync(
    string sourceRepo = null,
    string targetRepo = null,
    int? channelId = null,
    bool? sourceEnabled = null,        // New
    string sourceDirectory = null,     // New
    string targetDirectory = null);    // New

This provides a cleaner, more flexible API that supports both general subscription filtering and specific codeflow validation needs without code duplication.

Error Messages

Clear, actionable error messages help users understand and resolve conflicts:

A backflow subscription 'abc123' already exists for the same target repository and branch. 
Only one backflow subscription is allowed per target repository and branch combination.

Benefits

• Maintainable: Validation logic exists in one place
• Consistent: Same validation behavior across all entry points
• Clean API: Generic subscription querying without method duplication
• No breaking changes: Existing functionality remains unaffected

Fixes #5204.

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