fix(test): bound extension CLI subtest concurrency

[jimhoyd]

Problem and change

Release CI 36882765069 fails the macOS Node 24 extension policy test: validate is terminated at its 20-second child-process deadline, returning 143 instead of 0.

The outer test-file cap does not constrain concurrency:true subtest groups. The policy group launches 23 CLI cases at once, each potentially starting configuration workers. Bound all four CLI subtest groups in this file to two concurrent cases. Every case, assertion and timeout remains unchanged.

Fixes #1126.

Verification

• Focused macOS Node 24 extension tests: all 90 passed.
• npm run verify: passed on macOS Node 26, including 1,397 root tests, workspace verification and integration proofs.
• All PR checks passed.
• Full OS/Node matrix: the previously failing macOS Node 24 shard 3 passed. Remaining matrix jobs are still running; no failures reported so far.

Compatibility and security

Test scheduling only; no runtime, policy, sandbox, package or public behavior changes.

• Changes match the documented portable contract; docs/examples are updated where needed.
• Relevant tests and package checks pass, or limitations are explained above.
• No credentials, customer data or private project material are included.
• Runtime/starter changes remain aligned where applicable.