Make darc delete-channel work with the configuration repo

[Copilot]

Description

Implements configuration repository support for darc delete-channel behind the DARC_USE_CONFIGURATION_REPOSITORY environment variable, following the established pattern from delete-subscriptions.

Changes

• IConfigurationRepositoryManager: Added DeleteChannelAsync method
• ConfigurationRepositoryManager: Implemented deletion using existing DeleteModelInternalAsync infrastructure with YamlModelUniqueKeys.GetChannelKey
• DeleteChannelCommandLineOptions: Extended ConfigurationManagementCommandLineOptions instead of CommandLineOptions to inherit config repo flags:
  • --configuration-repository
  • --configuration-branch
  • --configuration-base-branch
  • --configuration-file
  • --no-pr
• DeleteChannelOperation: Routes to config repo manager when flag set, preserves API-based path for backward compatibility
• DeleteChannelOperationConfigRepoTests: Added test coverage for file operations, searching, and cleanup

Usage

# Delete channel via config repo
export DARC_USE_CONFIGURATION_REPOSITORY=true
darc delete-channel --name "test-channel" \
  --configuration-branch my-branch \
  --no-pr

# Without flag, uses existing API path (default)
darc delete-channel --name "test-channel"

The API path can be removed in future by deleting the else block in DeleteChannelOperation.ExecuteAsync.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Make darc delete-channel work with the configuration repo</issue_title>
<issue_description>Depends on #5480

Context

With the channel configuration moving to the configuration repo, we should make it so darc delete-channel modifies the repo config. During the transition period, this should be behind a flag in the command.

Goal

Modify the existing darc delete-channel command so it updates the default channel by modifying the configuration repo in whichever structure we agree on, using the available yaml models. We'll need to add a few flags to the command:

• Flag to target the repo instead of the API
• Flag to specify configuration repository
• Flag to specify which branch to commit the config changes to (if we already have a PR, or for scenario tests)
• Flag to specify which base branch to use for the PR creation
• Flag to specify we don't want to open a PR (for scenario tests)

This should be written so it makes it easy to remove the API part, and use the configuration repo as one and only way to modify the configuration
</issue_description>

Comments on the Issue (you are @copilot in this section)

@dkurepa The operation should look similar to `delete-subscriptions`, adding a method to the IConfigurationRepositoryManager and then implementing it. It should also have a test similar to `DeleteSubscriptionOperationConfigRepoTests`

• Fixes Make darc delete-channel work with the configuration repo #5505

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