feat: support Annotated wrappers for Inline and Externalized field markers

[myeolinmalchi]

Problem

AgentRef field detection currently relies on Inline[T] and Externalized[T] being visible as the outer annotation. In the common path, get_type_hints() strips outer Annotated[...] metadata, so declarations like Annotated[Externalized[T], ...] can already work incidentally.

However, the raw annotation fallback path and any future move to get_type_hints(..., include_extras=True) would preserve Annotated as the outer type. In those cases, is_inline_annotation(), is_externalized_annotation(), and get_wrapped_type() would no longer identify AgentRef field markers correctly.

Proposal

Add an internal helper that recursively unwraps outer typing.Annotated layers before AgentRef marker detection and wrapped-type extraction.

Expected supported declarations:

from typing import Annotated

class MyState(AgentRefState):
    step: Annotated[Inline[str], "metadata"]
    docs: Annotated[Externalized[list[dict]], "metadata"]
    nested: Annotated[Annotated[Externalized[list[str]], "a"], "b"]

Scope should stay intentionally narrow: use Annotated metadata only for marker detection compatibility, and do not interpret framework reducer metadata or other metadata values as AgentRef behavior in this change.

Implementation likely belongs in agentref/core/types.py, used by:

• is_inline_annotation()
• is_externalized_annotation()
• get_wrapped_type()

Add focused tests covering outer and nested Annotated wrappers around both Inline and Externalized markers, including the raw-annotation fallback path if practical.

Alternatives considered

Keep relying on get_type_hints() default behavior, which strips Annotated metadata today. This is fragile because it does not cover fallback behavior and would break if AgentRef later preserves extras intentionally.

Support Externalized[Annotated[T, ...]] only. That already works for marker detection because Externalized remains the outer type, but it does not address the more idiomatic Annotated[Externalized[T], ...] form.

Related framework

All frameworks; most relevant to LangGraph because LangGraph commonly uses Annotated for reducer metadata.

Related backend

All CAS backends; this is annotation handling and should not affect storage behavior.

Expected impact

Improves type annotation compatibility and future-proofs AgentRef against preserving Annotated metadata. Runtime behavior should remain unchanged for existing Inline[T] and Externalized[T] declarations.