[test-generator] Cover observability log rotation guardrails

[r3dbars]

Missing coverage

Recent observability work added ObservabilityLogRotation.rotateIfNeeded(...), and the fast runner already compiles the seam, but there was no direct behavior coverage for the rotation boundary, rename result, previous-generation replacement, or owner-only permissions. I kept this separate from open PR #1103 by adding a dedicated log-rotation test file instead of editing the adjacent log-writer tests.

Tests added

• Missing log files are no-op guardrails.
• Files exactly at the threshold stay active.
• Oversized JSONL logs move to the .1 generation by rename.
• A new rotation replaces the prior generation and tightens the rotated file to owner-only permissions.

Checks run

• bash run-tests.sh --filter ObservabilityLogRotation (12 passed)
• bash scripts/dev/agent-preflight.sh
• bash -n run-tests.sh
• bash -n scripts/entrypoints/run-tests.sh
• bash build-deps.sh --force (needed after build.sh --no-open reported stale deps)
• bash build.sh --no-open
• bash run-tests.sh (5232 passed)