[Content Addressable] Update gem build flags to include --content-addressable when building a singe Ruby ABI gem

[OughtPuts]

#9899

TLDR

Removes the responsibility of building a content addressable gem out of the --ruby-abi X.Y flag and into a new separate flag, --content-addressable.

Description

This PR:

• Adds a --content-addressable flag option to BuildCommand
• Maintains the --ruby-abi option in BuildCommand but this command is now only responsible for setting a ruby ABI at build time (instead of in the spec). All help text is rewritten to reflect this.
• ruby_abi is no longer passed through to Gem::Package.build - the final argument is now a boolean representing whether or not --content-addressable is provided.
• We still validate the platform, ruby version and rubygems version, but now only when --content-addressable is passed.
• the ruby_abi_compatible? method in Gem::ContentAddress is now no longer used (we use Gem::ContentAddress.eligible? instead so this has been removed).
• Other Gem::Package.build callsites were adjusted, e.g. builders.rb and helper.rb.

Tests

Relevant functionality tests:

• Tests added / updated in clean_spec, content_addressable_spec, test_gem_commands_build_command, test_gem_content_address, test_gem_package.

Other test updates:

• Changed the spec gate from >= 4.1.0.dev to >= 4.1.0.a to align with the production minimum.
• Skip content-addressable integration specs on prerelease Rubies because their temporary test gems require the format ~> X.Y.0, which Ruby versions such as 4.1.0.dev do not satisfy, preventing the tests from reaching their intended assertions.
• Removed Dir[File.join(@tempdir, "platformed_gem-2-*.gem")].first from test_content_addressable_produces_deterministic_content_address as this was hiding a small bug. We should return the relevant Dir from the lamda from the build each time and not take the first matching directory of the set.

Top Hatting:

The below has passed successfully for me locally ✅

1. From a local gem directory (but specifying the rubygems changes present on this branch) Build a gem using the content addressable flag e.g. gem build --content-addressable. Confirm a content addressable gem is built e.g. ca_test-1.0.0-1234abcd.gem.

2. Build a gem using the content addressable flag AND the ruby-abi flag because required_ruby_version is not present in the spec e.g. gem build --content-addressable --ruby-abi 4.0. Confirm a content addressable gem is built e.g. ca_test-2.0.0-1234abcd.gem. Confirm that the Ruby ABI version has been correctly set in the spec.

3. Build a standard gem without either flag e.g. gem build. Confirm that a gem is built e.g. ca_test-3.0.0.gem

[kou]

+1

Could you check the CI failure?