fix(payjoin-ffi): validate brew and llvm on macOS

[AdamuAbba]

Summary

• commit - d33bc3c
  • the commit adds an early guard inside the Darwin + non-Nix-shell if block with a descriptive error message when brew or llvm are not installed

Closes #1773

hey @xstoicunicornx when you've got some time i'd appreciate that review. Thank you.

AI assistance disclosure

I consulted GitHub copilot to understand the codebase and most of the code was then generated using copilot inline completion

Pull Request Checklist

Please confirm the following before requesting review:

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• I have read CONTRIBUTING.md and rebased my branch to produce hygienic commits.

[xstoicunicornx]

utACK d33bc3c

Reviewed and tested locally. Update is straightforward guard against missing mac os dependencies brew and llvm.

[caarloshenriq]

ACK d33bc3c

[benalleng]

uTACK d33bc3c