fix(typing): runtime NameError in DictBasedTypingSystem.construct_state, correct ActionSchema abstract signatures

[adawang1210]

Summary

DictBasedTypingSystem.construct_state() crashed with NameError: name 'State' is not defined whenever it was actually called -- State is only imported under TYPE_CHECKING in burr/core/typing.py. This is reachable from the public API: ApplicationBuilder.with_state(<non-State object>) with a dict-based typing system hits this path (application.py calls self.typing_system.construct_state(state)). Additionally, ActionSchema's abstract methods were declared without self, contradicting every concrete implementation (DefaultSchema, the pydantic integration), and an unused Self import fallback was dead code.

Changes made

• burr/core/typing.py:
  • construct_state now imports State locally at runtime (module-level would be circular -- state.py imports from typing.py), fixing the NameError.
  • ActionSchema.state_input_type / state_output_type / intermediate_result_type abstract declarations now take self, matching all implementations.
  • Removed the unused try: from typing import Self fallback (never referenced).
• tests/core/test_typing.py (new): covers DictBasedTypingSystem behavior including the previously-crashing construct_state, a conforming ActionSchema subclass, ABC enforcement for incomplete subclasses, and DEFAULT_SCHEMA semantics.

How to test

pytest tests/core/test_typing.py

Repro of the bug on main:

from burr.core.typing import DictBasedTypingSystem
DictBasedTypingSystem().construct_state({"a": 1})  # NameError before this fix

Checklist

• Tests added or updated
• All tests pass locally (pytest tests/ --ignore=tests/integration_tests: 577 passed, 5 skipped)
• Commits signed off (git commit -s)
• Pre-commit hooks pass (black, isort, flake8)
• CONTRIBUTING.md guidelines followed