Fix bundler version parsing for RubyGems/Bundler 4.x output format

[tnikolova82]

Summary

Fixes a hard staging failure that broke CI on #1171 (the automated rubygems 3.4.19 -> 4.0.22 dependency bump):

**ERROR** Unable to add bundler path to default environment: failed to determine bundler version from output: 4.0.22 (2026-09-30 commit ff2bd50)
Failed to run all supply scripts: exit status 15

Root cause

GetBundlerVersion() (src/ruby/versions/ruby.go) parses the output of bundle version with a regex requiring a literal "Bundler version " prefix:

re := regexp.MustCompile(`Bundler version (\d+\.\d+\.\d+) .*`)

This matches the legacy output format (e.g. "Bundler version 2.7.2 (2025-09-09 commit b463ced1459)"). As of RubyGems/Bundler 4.x, that prefix has been dropped - the command now just prints the bare version number instead (e.g. "4.0.22 (2026-09-30 commit ff2bd50)"). The regex finds zero matches against the new format, so GetBundlerVersion() returns an error, and the whole supply script aborts (exit status 15) - breaking every integration test that needs bundle to resolve its version (4 failures in #1171's CI run: vendor_bundle, custom_gemfile, Offline, MultiBuildpack/.../binary_buildpack).

This is a real, 100%-reproducible functional regression exposed by the version bump - not CI flakiness.

Fix

• Extracted the parsing logic into a small, directly unit-testable pure function, parseBundlerVersion(output string) (string, error).
• Made the "Bundler version " prefix optional in the regex ((?:Bundler version )?(\d+\.\d+\.\d+)), so both the legacy and new output formats parse correctly.

Test plan

• New TestParseBundlerVersion (src/ruby/versions/ruby_internal_test.go) with 5 cases: legacy prefixed format, new unprefixed format (the exact string from the CI failure), a bare version number, unrecognized output, and empty output.
• go test ./src/ruby/versions/... - all 37 existing Ginkgo specs + 5 new test cases pass.
• go build ./..., go vet ./... - clean.

Relationship to #1171

This is a prerequisite fix for #1171 (the rubygems bump) to be mergeable - recommend rebasing/re-running #1171's CI once this lands.