Auto-trigger release pipeline on main merge and check npm versions before publish

[Yuchao Yan (msyyc)]

Changes

• Auto-trigger release pipeline: The release pipeline now triggers automatically when changes are merged to the \main\ branch, removing the need for manual triggers.
• Pre-publish npm version check: Added a step that runs early in the pipeline (before installing dependencies or running tests) to verify that the current package versions are not already published to npm. This avoids wasting time on builds and tests when the versions are already released.
  • Checks @azure-tools/typespec-python\
  • Checks @autorest/python\
• Updated README: Reflects the new automatic release behavior.