test: stabilize release preflight fixtures

[cbusillo]

Summary

• update active-session guidance test for the current isolated-worktree copy
• seed realistic auto-review identity fields in the idle observer fixture
• reset the skipped-review marker precondition so the test is deterministic under full nextest

Validation

• cargo test --manifest-path code-rs/Cargo.toml -p code-core active_sessions::tests::model_notice_gives_concurrent_editing_guidance -- --nocapture
• cargo test --manifest-path code-rs/Cargo.toml -p code-tui background_review_observe_idle_renders_notice_and_post_turn_input_from_agent_result --features test-helpers -- --nocapture
• cargo test --manifest-path code-rs/Cargo.toml -p code-tui duplicate_skipped_background_review_does_not_mark_snapshot_reviewed --features test-helpers -- --nocapture
• ./build-fast.sh
• CODE_HOME=$(mktemp -d /tmp/code-pre-release-code-home.XXXXXX) CODEX_HOME=$CODE_HOME ./pre-release.sh

Agent review: two read-only reviewers found no issues and agreed these are fixture/isolation fixes.