fix: harden Codex backend parity across runtime and validation

[VasuBansal7576]

Description

Hardens the backend/runtime path that Codex was exposing more aggressively than other models. This PR focuses on validation, rerun/input propagation, worker handoff, result visibility at the event layer, stop behavior, and parity regression coverage.

Stacked on top of PR1: aden-hive#6830

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Related to aden-hive#6741
Related to aden-hive#6817

Changes Made

• block stage/run on failed validation and fix the validation subprocess entrypoint
• preserve structured run/rerun inputs and worker handoff state across backend flows
• fix stale phase/tool/output state, result handoff overwrites, terminal stop handling, and no-progress churn detection
• improve worker health reporting and add Codex parity / Codex-vs-control regression coverage

Testing

Ran the following tests locally:

• cd core && uv run pytest tests/test_codex_control_parity.py tests/test_codex_parity_gate.py tests/test_codex_planning_phase.py tests/test_event_loop_node.py tests/test_queen_lifecycle_validation.py tests/test_queen_orchestrator.py tests/test_session_manager_worker_handoff.py tests/test_session_manager_worker_validation.py tests/test_validate_agent_path.py tests/test_worker_monitoring_tools.py framework/server/tests/test_api.py -q
• cd core && ruff check .
• Manual testing performed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Screenshots (if applicable)

N/A

[github-actions]

PR Closed - Requirements Not Met

This PR has been automatically closed because it doesn't meet the requirements.

PR Author: @VasuBansal7576
Found issues:
Problem: The PR author must be assigned to the linked issue.

To fix:

1. Assign yourself (@VasuBansal7576) to one of the linked issues
2. Re-open this PR

Exception: To bypass this requirement, you can:

• Add the micro-fix label or include micro-fix in your PR title for trivial fixes
• Add the documentation label or include doc/docs in your PR title for documentation changes

Micro-fix requirements (must meet ALL):

Qualifies	Disqualifies
< 20 lines changed	Any functional bug fix
Typos & Documentation & Linting	Refactoring for "clean code"
No logic/API/DB changes	New features (even tiny ones)

Why is this required? See aden-hive#472 for details.

[VasuBansal7576]

Closing this in favor of the single consolidated PR: aden-hive#6831