fix: resolve meta block type annotations

[TZZheng]

Summary

• replace the undefined AgentState annotation with object, matching the helper's duck-typed use of agent instances
• import Any and Dict so the existing postponed annotations can be resolved at runtime
• add a regression test that resolves type hints for all four affected helpers

Validation

• .venv/bin/python -m pytest tests/test_meta_block.py -q — 181 passed
• git diff --check canonical/main...HEAD — passed
• independent read-only review — PASS, no blockers

Risk

This is annotation-only production code plus a focused regression test; runtime control flow is unchanged.

Fixes #752