ref(install): Read the JS SDK loader registry with json.tool instead of cat

[oioki]

setup-js-sdk-assets.sh read _registry.json from the web container with cat. Use python3 -m json.tool instead: the sentry image is moving to a distroless base without coreutils. The file is only consumed by jq, and json.tool re-emits the same JSON (verified: identical parsed content and extracted SDK versions). python3 isn't a sentry subcommand, so the entrypoint execs it directly, like cat.

Covered by _unit-test/js-sdk-assets-test.sh.

Prep for moving the sentry image to a distroless base.

[github-actions]

Coverage Results 📊

✅ 22 passed | Total: 22 | Pass Rate: 100% | Execution Time: 10m 51s

📊 Comparison with Base Branch

Metric	Change
Total Tests	—
Passed Tests	—
Failed Tests	—
Skipped Tests	—

✨ Test counts unchanged from base.

All tests are passing successfully.

✅ Patch coverage is 100.00% (no changed executable lines found; target 50%).
Project statement coverage is 95.54% (unchanged from base (49e367f) to head (9a85fcc)).

Coverage diff

@@            Coverage Diff             @@
##        master     #4540       +/-##
==========================================
  Coverage    95.54%    95.54%        —%
==========================================
  Files            5         5         —
  Tracked lines       336       336         —
  Branches         0         0         —
==========================================
  Hits           321       321         —
  Misses          15        15         —
  Partials         0         0         —

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