Improve Pixel logout cleanup and raw responses

[springwq]

Summary

• Remove locally stored Pixel credentials when remote revocation fails, while preserving the failure exit status and reporting local cleanup and remote revocation separately.
• Add fm pixel request get --raw for verbatim response output, reject conflicting --json usage and duplicate /rails prefixes, and document CSV exports and logout behavior.
• Add regression coverage for revocation failures, environment credentials, raw output, API errors, and request validation. Verified on latest main: 113 tests, 585 assertions, zero failures/errors (6 platform skips); RuboCop and git diff --check pass. Production API requests were not exercised.