Add channel test providers for Email, Slack, Teams, and Webhook
- Implemented EmailChannelTestProvider to generate email preview payloads. - Implemented SlackChannelTestProvider to create Slack message previews. - Implemented TeamsChannelTestProvider for generating Teams Adaptive Card previews. - Implemented WebhookChannelTestProvider to create webhook payloads. - Added INotifyChannelTestProvider interface for channel-specific preview generation. - Created ChannelTestPreviewContracts for request and response models. - Developed NotifyChannelTestService to handle test send requests and generate previews. - Added rate limit policies for test sends and delivery history. - Implemented unit tests for service registration and binding. - Updated project files to include necessary dependencies and configurations.
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