fix(vendor): vendor pure-ruby (?platform=ruby) gems, gate on the purl not the staging dir - #172

Problem

Vendoring a pure-ruby gem whose production patch purl is platform-qualified — pkg:gem/activestorage@7.0.2.2?platform=ruby — was refused with platform_gem_unsupported ("installed dir gem does not equal activestorage-7.0.2.2"). The ruby platform is the DEFAULT, portable RubyGems platform, not a native build, so this refusal is wrong.

The gem vendor gate used installed_dir.file_name() != "<name>-<version>" as a proxy for "platform-specific build". That proxy breaks on the registry auto-fetch ladder: a lockfile-resolvable-but-not-installed gem is staged into a private tempdir named literally gem (registry_fetch::fetch_gem), so dir_name is gem and every pure-ruby auto-fetched gem tripped the gate.

What #157 already fixed vs. what remained (verified against current main, real production API + rubygems.org, SOCKET_NO_CONFIG=true):

• Installed gem (task's exact repro — bundle config set --local path vendor/bundle, bundle install, scan --mode vendored): fix(vendor): resolve installed packages via the qualified-purl-aware lookup #157's find_packages_for_rollback maps the qualified ?platform=ruby purl back to the installed base dir, so no auto-fetch happens and it vendors cleanly — 8/8 runs, applied: 1, exit 0. fix(vendor): resolve installed packages via the qualified-purl-aware lookup #157 fixed this runtime path.
• Auto-fetch gem (lockfile-resolvable but not installed where the crawler scans — fresh clone, out-of-tree install): still hit the bug on current main, refusing with the identical platform_gem_unsupported error (event vendor_fetched_missing → staged gem dir → refused). This is what this PR fixes.

Fix

Gate on the authoritative signal — the purl's own ?platform= qualifier — instead of the staging dir name:

• ?platform=ruby (and a bare, unqualified purl) is the portable build and vendors normally.
• Only a genuine native platform (x86_64-linux, arm64-darwin, java, x64-mingw32, …) is refused, with the same clear platform_gem_unsupported code.
• Defense in depth: a locally-resolved platform-suffixed install dir (<name>-<version>-<suffix>) is still refused (preserving the downstream invariant that platform installs never reach the copy/lock edit). The auto-fetch gem dir is not such a suffix, so it passes.

Adds a small reusable purl_qualifier(purl, key) to utils::purl (the other ecosystems only ever strip qualifiers) rather than hand-rolling the parse. Change is scoped to the gem vendor path.

Test

• New hermetic unit tests in the gem vendor module (RED before / GREEN after — both fail on the old gate, the first reproducing the exact installed dir gem does not equal rack-3.2.6 symptom):
  • test_platform_ruby_gem_from_autofetch_staging_dir_vendors — a ?platform=ruby gem from a gem-named auto-fetch staging dir now vendors successfully.
  • test_refuses_native_platform_qualifier — a native ?platform=x86_64-linux purl is still refused even from a clean leaf dir.
  • test_refuses_platform_suffixed_dir (existing) stays green via the defense-in-depth dir check.
  • test_purl_qualifier — unit coverage for the new helper.
• Real end-to-end (rebuilt binary, SOCKET_NO_CONFIG=true, live prod + rubygems.org):
  • The previously-broken auto-fetch scenario now vendors cleanly 4/4 runs (applied: 1, exit 0), via vendor_fetched_missing → vendor_prebuilt_downloaded.
  • Installed-gem scenario still clean 8/8.
  • Delivery proof: a cold, frozen bundle install from the committed .socket/vendor/ tree succeeds (exit 0) and bundle info activestorage resolves 7.0.2.2 from the vendored (patched) path.
• cargo build + cargo clippy (both crates) clean; the only clippy warnings are pre-existing doc_lazy_continuation in e2e_vendored_production.rs (untouched).

Note (separate, pre-existing, out of scope): with the default --vendor-source auto/service, the patch-service gem-stub-gemspec for activestorage omits s.summary, which RubyGems' validation rejects on bundle install. This is a converter/service-side artifact defect, not in the gem vendor CLI path — it affects the installed and auto-fetch paths equally and was simply never reached before because vendoring failed first. Delivery via the local-build stub (a valid gemspec) works, as shown above. The live-ignored gem_bundler_vendored_known_platform_defect e2e leg will now take its built-in "vendor succeeds" branch; promoting it to a full frozen-install delivery proof is a follow-up gated on that stub-summary fix.

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