Fall back to lockfile version when BUNDLE_VERSION is "lockfile"

[hsbt]

What was the end-user or developer problem that led to this PR?

Bundler treats "lockfile" as a special value for the version setting, meaning "use the version recorded in BUNDLED WITH".

BundlerVersionFinder was passing the raw string to Gem::Version.new and raising ArgumentError both from the env var added in #9538 and from .bundle/config.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[Copilot]

Pull request overview

Fixes Gem::BundlerVersionFinder to recognize the special value "lockfile" for the BUNDLE_VERSION setting (and BUNDLE_VERSION env var added in #9538). Previously, the literal string "lockfile" was passed to Gem::Version.new, raising ArgumentError. Now the finder skips assigning that sentinel and falls through to lockfile_version.

Changes:

• Capture bundle_config_version once into bcv and short-circuit on "system" as before.
• Skip using bcv as the version when its value is the sentinel "lockfile", so the lockfile's BUNDLED WITH value is used.
• Add tests covering both the BUNDLE_VERSION=lockfile env var path and the .bundle/config BUNDLE_VERSION: "lockfile" path.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/rubygems/bundler_version_finder.rb	Treat "lockfile" as a sentinel and fall back to the lockfile-recorded version instead of passing the literal to Gem::Version.new.
test/rubygems/test_gem_bundler_version_finder.rb	Adds two tests covering the new sentinel behavior via env var and via global bundle config.

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