[OskarEichler]

Summary

Duplicate parser options before extracting the decoder.

Reproduction and verification

Construct JSON response middleware twice from the same parser_options hash containing :decoder: baseline deletes the decoder and the second instance falls back to JSON. A frozen options hash raises FrozenError. Both work after the fix.

• Individual patch: 9 focused Faraday checks passed (json); all 639 existing RSpec examples pass; changed-file RuboCop and Ruby syntax pass.
• Combined installed-release-based branch: 639 existing examples, 1,124 focused checks, full RuboCop (75 files), YARD checks and gem packaging pass.
• External faraday-net_http adapter suite: 386 existing examples, zero failures with the combined fixes.
• Ruby 4.0.6 through rbenv. No production or live external HTTP operations.

Limitations

No test/spec files were added or changed because the originating repository explicitly forbids this. Reproductions ran outside the repository. This differs from Faraday's requested regression-test contribution convention and is disclosed for maintainer review. Other Ruby versions/platforms were not run locally. This PR includes only the focused source change; the other fixes and the consumer's separately verified merged JSON decoder patch #1687 are not added here.

Breaking-change notes

No API changes. Initialization stops mutating caller options and snapshots their top-level values.

[olleolleolle]

This change looks good.

The note about the repo not accepting tests read a little strong.