Add configuration repository support to darc add-channel

[Copilot]

Implements configuration repository mode for darc add-channel to add channels via git repository instead of API calls, mirroring the existing add-subscription implementation.

Changes

Command Interface

• AddChannelCommandLineOptions now inherits ConfigurationManagementCommandLineOptions, exposing:
  • --configuration-repository - Target repo URI
  • --configuration-branch - Working branch (auto-generated if unspecified)
  • --configuration-base-branch - Base for PR creation
  • --configuration-file - Override default file path
  • --no-pr - Skip PR creation (commit only)
• Mode controlled by DARC_USE_CONFIGURATION_REPOSITORY environment variable

Core Implementation

• AddChannelOperation checks for existing channels in API before committing (prevents duplicates during transition)
• ConfigurationRepositoryManager.AddChannelAsync handles:
  • Branch preparation and validation
  • YAML file fetch/parse/update with ChannelYamlComparer ordering
  • Duplicate detection in target file
  • Commit and PR creation

Testing

• AddChannelOperationConfigRepoTests validates file creation, append behavior, duplicate detection (API and YAML), and YAML format
• Uses ConfigurationManagementTestBase with real git repo (not mocked)

Usage

export DARC_USE_CONFIGURATION_REPOSITORY=true
darc add-channel --name ".NET 11" --classification "product"

# To specific branch without PR
darc add-channel --name ".NET 11" --classification "product" \
  --configuration-branch "existing-branch" --no-pr

API mode remains default (backward compatible). Configuration mode creates/updates files at configuration/channels/{normalized-name}.yml.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Make darc add-channel work with the new 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 add-channel can modifies the repo config. During the transition period, this should be behind a flag in the command.

Goal

Modify the existing darc add-channel command so it adds the 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)
• Flag to overwrite default file for the channel, and put the new channel in the provided file

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>

<agent_instructions>Look at the implementation for AddSubscriptionOperation. Also it's part in the ConfigurationRepository Manager, they should look quite similar</agent_instructions>

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

@dkurepa The implementation for this operation should look similar to the AddSubscriptionAsync one in the ConfigurationRepositoryManager.

We should be able to reuse most of the methods for fetching the file, commiting it, etc..</comment_new>

• Fixes Make darc add-channel work with the new configuration repo #5500

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[dkurepa]

@copilot also make sure to add tests like we have for AddSubscriptionOperation

[Copilot]

@copilot also make sure to add tests like we have for AddSubscriptionOperation

Tests have been added following the same pattern as AddSubscriptionOperationConfigRepoTests. All 5 tests are passing:

• AddChannelOperation_WithConfigRepo_CreatesChannelFile
• AddChannelOperation_WithConfigRepo_AppendsToExistingFile
• AddChannelOperation_WithConfigRepo_FailsWhenChannelAlreadyExistsInApi
• AddChannelOperation_WithConfigRepo_FailsWhenChannelAlreadyExistsInYamlFile
• AddChannelOperation_WithConfigRepo_FileContentIsValidYaml

Commit: c178bc0