The problem

create_github_release raises whenever release_notes_file_path is omitted:

create_github_release_action.rb:17:in `gsub!': [!] can't modify frozen String: "" (FrozenError)

release_notes_file_path is declared optional: true, so omitting it is a supported path — but the '' fallback it lands on is a frozen literal, because the file sets frozen_string_literal: true. The next line then calls gsub! on it (the rewrite that turns bracketed GitHub PR/issue URLs into shorthand) and blows up.

File.read returns an unfrozen string, which is why the branch that does read a notes file has never hit this — and why every existing caller in the wild works. The optional path has simply never been exercised.

What this PR does

• Uses the non-mutating gsub and reassigns. The regex and its behaviour are unchanged.
• Adds spec/create_github_release_spec.rb, which the action didn't have:
  • the no-notes path, covering the regression directly;
  • the URL-rewrite behaviour, pinning the one line this PR touches so gsub + reassignment is proven equivalent to gsub!.

Checklist before requesting a review

• Run bundle exec rubocop to test for code style violations and recommendations.
• Add Unit Tests (aka specs/*_spec.rb) if applicable.
• Run bundle exec rspec to run the whole test suite and ensure all your tests pass.
• Make sure you added an entry in the CHANGELOG.md file to describe your changes under the appropriate existing ### subsection of the existing ## Trunk section.
• If applicable, add an entry in the MIGRATION.md file to describe how the changes will affect the migration from the previous major version and what the clients will need to change and consider.