[JS] fix: #247 #635 Add fallthrough for empty result Task Module Fetch and tests

[corinagum]

Linked issues

closes: #635
closes: #247

Details

• A behavior that was missing from Task Modules was handling submit "continue" scenario when the handler does not return a TaskModuleResponse or string. This ensures that the expected response (empty 200) is sent even if there is nothing new to render/send.
• Modify so that Teams events check for channelId using Channels enum, not 'msteams' string
  • This included minor refactor for Message Extension and Application as well
• Add Task Module tests to get the file to 80% coverage

Attestation Checklist

• My code follows the style guidelines of this project

• I have checked for/fixed spelling, linting, and other errors

• I have commented my code for clarity

• I have made corresponding changes to the documentation (we use TypeDoc to document our code)

• My changes generate no new warnings

• I have added tests that validates my changes, and provides sufficient test coverage. I have tested with:

  • Local testing
  • E2E testing in Teams

• New and existing unit tests pass locally with my changes

Additional information

Thank you @th1nkful for the original PR, which I modified to match Bot Framework protocol :)