Expose performance test output reports to scripts

[scriptonist]

Summary

• propagate the host-provided performance test report through Runtime script contexts
• expose it as pm.performanceTest.output.report.json through the supporting Sandbox release
• preserve complete API absence when no report is supplied and inherit the context for nested requests
• advertise the native report capability for agent negotiation

Verification

• 572 unit tests passing, 10 pending
• 1902 integration tests passing, 33 pending; one unrelated localhost:3000 environment-only failure
• focused parent, nested-request, absence, and capability tests passing
• lint and diff checks clean