fix: stop test fixtures from deleting the repository's dependencies

Four fixtures linked the installed dependencies into a throwaway root with a
single directory symlink at <fixture>/node_modules, then ran pnpm inside that
root. pnpm does not recognise the modules directory it finds there and purges
it; with CI=true it does so without a prompt. The purge followed the symlink and
deleted the repository's own node_modules mid-run, so a test suite uninstalled
the workspace it was running in. That is what produced the cascading,
file-unrelated failures a full test:unit run reported, and it happened twice
while running the suites for the adapter re-review.

scripts/lib/fixture-node-modules.ts replaces all four sites: node_modules is a
real directory whose entries are individual symlinks, so a recursive delete
unlinks the fixture's own links instead of walking through one link into the
shared tree. Resolution is unchanged.

tests/unit/fixture-node-modules.test.ts performs the exact recursive delete pnpm
performs and asserts the source tree survives, and check:adapter-inventory now
fails on any reintroduction of the directory-symlink form — verified by putting
the old line back and watching the gate reject it.

A full tests/unit + tests/integration run now leaves the dependencies intact.
removal-fixture, supply-chain and security-followup-archive, the three suites
that had to be excluded before, pass in that run.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/unit/ci-artifact-contract.test.ts

@@ -19,6 +19,7 @@ import {
   validateCiArtifact,
 } from "../../scripts/lib/ci-artifact-validator.ts";
 import { writeCiGateLogAtomic } from "../../scripts/lib/ci-gate-log.ts";
+import { linkFixtureNodeModules } from "../../scripts/lib/fixture-node-modules.ts";
 import { captureCiCandidateArchive, verifyCiCandidateArchive } from "../../scripts/lib/ci-candidate-archive.ts";
 import {
   CANDIDATE_ARCHIVE_USAGE,
@@ -2131,7 +2132,7 @@ async function ensureProviderBaseFixture(): Promise<string> {
     recursive: true,
   });
   await rm(path.join(root, "artifacts/release"), { recursive: true, force: true });
-  await symlink(path.join(sourceRoot, "node_modules"), path.join(root, "node_modules"), "dir");
+  await linkFixtureNodeModules(root, sourceRoot);
   const git = spawnSync("git", ["show", "-s", "--format=%H%n%ct", "HEAD"], {
     cwd: sourceRoot,
     encoding: "utf8",