fix(elixir-sdk): use PR instead of direct push for spec regeneration

The publish-elixir-sdk workflow has been failing daily since March 30
because it tries to push directly to main, which is a protected branch
requiring PRs.

Split the workflow into two jobs:
- regenerate: runs on schedule/dispatch, creates a PR with changes
- publish: runs on push to main, publishes to hex.pm if version bumped

This ensures the auto-generated code goes through the normal PR flow
while still auto-publishing when changes land on main.