fix(deps): keep a single Strapi in the workspace

[derrickmehaffy]

What changed and why

Since #242 moved the playgrounds to Strapi 5.57, CI has been failing intermittently on unrelated PRs. #248, #253 and #255 each failed one job: e2e_memory, e2e_redis or boot_smoke on Node 26. All three failed with:

TypeError: strapi.ai.admin.isEnabled is not a function
    at Object.isEnabled [as register] (…/@strapi+content-type-builder@5.52.0_…)
    at Object.register (…/@strapi+core@5.57.0_…)

Cause: the lockfile held two complete Strapi installs, 5.52.0 and 5.57.0.

• The 5.52 copy was kept alive by the memory provider's auto-installed @strapi/strapi ^5.0.0 peer, still locked at 5.52.0. Dependabot moves the playgrounds' pinned versions, not a locked peer.
• Strapi core resolves its built-in plugins (content-type-builder, content-manager, upload, …) through the workspace's hoisted node_modules, not through its own dependencies.
• So a 5.57 app sometimes loaded the 5.52 content-type-builder, which calls an API 5.57 no longer has.

Fix: pnpm dedupe. Every @strapi/* package is now at 5.57.0 only.

• It adds no package version that wasn't already in the lockfile; I compared the full package sets.
• It removes 158 duplicate versions (−3046/+245 lines).
• No package.json range changes, so nothing reaches users.

Guard: a new scripts/check-single-strapi.cjs, run by test:deps, fails when @strapi/strapi or @strapi/core appear at more than one version, and says to run pnpm dedupe.

• On the previous lockfile: FAIL @strapi/strapi: 2 versions (5.52.0, 5.57.0).
• On this one: ok.

Tests

• test:boot: both playgrounds boot under real Node resolution. This is the job that failed on fix(admin): purge the open entry from the edit view, and let the purge button be used twice #253.
• test:e2e:memory: 135/135, run twice.
• test:e2e:redis: 135/135.
• test:admin: 13/13.
• test:lint, test:deps: green.

Notes for the reviewer

• Merge this first. The open fix PRs that went red on this flake (fix: treat options set to undefined as omitted #248, fix(admin): purge the open entry from the edit view, and let the purge button be used twice #253, fix(purge): purge cached types that embed an uncached content type #255) should go green once rerun on top of it.

Checklist

• Tests: a guard that fails on the broken lockfile.
• Conventional commit subjects, branched from main.