fix(pinia-orm): fall back to strong refs when WeakRef is unavailable

[CodeDredd]

Bug

The default query cache (WeakCache) constructs WeakRef instances on every set(). Environments without WeakRef support — notably Cloudflare Workers — throw WeakRef is not defined as soon as anything is cached, which breaks Nuxt SSR on that platform (#1949).

Fix

WeakCache#set() now falls back to an internal StrongRef (same deref() shape) when WeakRef is not available. Degraded mode holds values strongly — acceptable for short-lived worker isolates, and far better than crashing. No behavior change where WeakRef exists.

Tests

New test stubs WeakRef away (vi.stubGlobal) and verifies set/get/has/iteration still work. Full suite green.

fixes #1949

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.40%. Comparing base (018acee) to head (6c3d0a5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2031   +/-   ##
=======================================
  Coverage   99.40%   99.40%           
=======================================
  Files          82       82           
  Lines        3025     3025           
  Branches      533      533           
=======================================
  Hits         3007     3007           
  Misses         15       15           
  Partials        3        3           

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