[OskarEichler]

Summary

Clear the cached middleware app when duplicating a builder.

Reproduction and verification

Make a request on a test-adapter connection, duplicate it, replace the copy's adapter and make another request. Baseline still calls the original adapter. The fix builds the copied app and leaves the original unchanged.

• Individual patch: 6 focused Faraday checks passed (builder); 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

Restores the editable-copy contract described in #55; no new API.

[iMacTia]

Closing in favour of #1718 which also comes with tests