Add Result Builder API for declarative message construction

[diegotl]

Summary

This PR adds a result builder API for declarative message construction and comprehensive integration tests for end-to-end verification.

Result Builder API

Following SlackKit's architecture pattern, all builders use Swift's @resultBuilder attribute for clean, type-safe DSL syntax.

New Builder Files

1. Message+Builder.swift - @AttachmentBuilder for building attachment arrays
2. Attachment+Builder.swift - @FieldBuilder and @ActionBuilder for fields and actions
3. Props+Builder.swift - @PropsBuilder for dynamic properties
4. Confirmation+Builder.swift - @ConfirmationBuilder for confirmation dialogs

Key Features

• Message Builder: Declarative attachment construction

  Message(text: "Deployment complete!") {
      Attachment(color: "#36a64f", title: "Build #123") {
          Field("Branch", value: "main")
          Field("Commit", value: "abc123")
      }
  }

• Props Builder: Dynamic properties with type-safe convenience functions

  Props(card: "Info") {
      Property("app_id", value: "12345")
      Property("environment", value: "production")
  }

• Confirmation Builder: Custom button configurations

  Confirmation(text: "Deploy to production?") {
      ConfirmButton(text: "Deploy", style: "primary")
      DenyButton(text: "Cancel")
  }

• Conditional Logic: Native if/else and for-in support

• Convenience Functions: Field(), Button(), Property(), ConfirmButton(), DenyButton()

Integration Tests

Added comprehensive end-to-end integration tests that send actual messages to Mattermost via Incoming Webhooks.

Test Coverage (47 tests)

• Simple Text Messages: Plain text, markdown formatting, custom username/icons
• Attachments: Single/multiple, with fields, author, images, thumbnails, footer
• Action Buttons: Single/multiple buttons with different styles
• Confirmation Dialogs: Buttons that require user confirmation
• Builder API: Result builders with conditionals and loops
• Message Priority: Important and urgent priority levels
• Props and Card: Custom metadata and sidebar content
• Special Characters: Emojis, unicode, code blocks
• Complex Messages: Deployment notifications, error alerts, feature announcements

Running Integration Tests

Tests are disabled by default. Set environment variables to run:

MATTERMOST_INTEGRATION_TESTS=1 MATTERMOST_TEST_WEBHOOK_URL="https://your-server.com/hooks/xxx" swift test

All tests are serialized with delays between requests to avoid rate limits.

Documentation Updates

• README.md updated to showcase the Result Builder API
• Added examples for Props and Confirmation builders
• Builder API Reference section with all builders
• Cross-reference to SlackKit companion package added

Test plan

• All existing tests pass
• New result builder tests pass (20 tests)
• Conditional logic (if/else) works correctly
• For-in loops work for dynamic content
• Encoding/decoding maintains data integrity
• All integration tests pass (47 tests)
• Integration tests verified against real Mattermost webhook

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