test: preserve machine gate settings when pinning run models

[santoshkumarradha]

Three run-model tests replaced the complete profile after the shared fixture disabled machine load and memory gates. On a busy host this silently restored the default load limit, so the scripted jobs never started and timed out.

Apply model pins through the settings API, preserving the fixture's resource settings. This only changes test setup. Partially addresses #1525.

Spark evidence: unchanged base 837b2b0 reproduces the load refusal in 61.486 seconds; with the fix all three affected tests pass in 5.736 seconds while sharing the same loaded host. Final make pr-ready passed on Spark at 53877754b, including the complete affected cmd/codeaf suite in 233.832 seconds. CI run 36334534140 is green. No product runtime changed, so this fixture correction requires no separate model-driven workflow. Separate cleanup-race failures are investigated in #1616 and #1612; they are not masked by this change.

[santoshkumarradha]

Superseded by the consolidated draft #1627. Exact reviewed head 53877754b0835743d5ff4b4572766222bffb6967 is included in published Santosh/dev integration ef1ba797ab6adbf4ff519d5808316d07d982c002; ancestry was verified after push. Source findings, validation, recordings and limitations are preserved in #1627. This closes the separate review destination; dev/main were not merged and final aggregate acceptance is still pending.