[deliver] keep an explicitly set platform when a pkg is present - #30224

Credit to @Loupv for the detailed report and root-cause analysis in #30213.

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I see several green GitHub Action builds in the "All checks have passed" section of my PR.
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.
• I've added or updated relevant unit tests.

Motivation and Context

Deliver::DetectValues#find_platform sets platform to osx whenever pkg is set. pkg defaults to any *.pkg in the current directory, so a metadata-only iOS lane run from a folder that happens to contain a pkg gets its explicit platform: "ios" replaced with osx. deliver then creates a macOS version on the app record, skips release notes as "first version of the app", and exits 0.

Resolves #30213

Description

The fix suggested in the issue (options[:platform] ||= 'osx') doesn't work on its own: platform has default_value: "ios", so options[:platform] is never nil and ||= would never pick osx. That would break macOS uploads that rely on the pkg to infer the platform, e.g. upload_to_app_store after gym builds a pkg.

Instead, find_platform now only infers osx from the pkg when the user hasn't set a platform. "Set" means passed in directly, set through DELIVER_PLATFORM, or set in the Deliverfile, which are the non-default sources Configuration#fetch reads from.

The ipa branch is unchanged, and it still takes precedence when both an ipa and a pkg are present.

Testing Steps

Added find_platform specs in deliver/spec/detect_values_spec.rb. Each one builds real Deliver::Options in a temp dir that contains a MyApp.pkg or MyApp.ipa:

• pkg present, no platform -> osx
• pkg present, platform: 'ios' passed -> ios (fails on master)
• pkg present, DELIVER_PLATFORM=ios -> ios (fails on master)
• pkg present, platform('ios') in Deliverfile -> ios
• ipa present, no platform -> ios

I also tried the plain ||= change, and the "no platform -> osx" example fails with it.

Run in a ruby:3.3 container:

• bundle exec rspec deliver/spec/detect_values_spec.rb: 14 examples, 0 failures
• bundle exec rspec deliver: 243 examples, 0 failures
• bundle exec rubocop on the two changed files: no offenses
• bundle exec rspec from the root: 7861 examples, 2 failures. Both are in fastlane/spec/plugins_specs/plugin_generator_spec.rb (the generated plugin's rubocop/rake checks) and fail the same way on unmodified master in the same container.