[AK] Standardize pluggable-backend factory pattern + BYO extension

[amithad]

Problem

AK's backend-selection factories have drifted apart. Across InputGuardrailFactory/OutputGuardrailFactory, Trace.get(), SessionStoreBuilder, ThreadStoreBuilder, the multimodal _build_driver, and the AWS response-store handler:

• Unknown type is handled inconsistently — guardrail/trace raise a bare Exception; session/thread/response stores silently fall back to an in-memory default (a production footgun).
• The "install the extra" hint is ad hoc — only SessionStoreBuilder's valkey branch has it, though redis/dynamodb/cosmos/firestore need extras too.
• No shared, typed config error — bare raise Exception(...) in several places.
• No bring-your-own (BYO) path — a user on an unsupported backend (e.g. Datadog for tracing) can't plug in their own implementation without forking AK.

Goal

One house pattern for every pluggable surface:

• Built-ins resolved via if/elif + real imports (grep/mypy/refactor-safe).
• A shared resolve_dotted(path, base=…) else branch on every surface → first-class BYO (point type at a dotted path to your own subclass, e.g. acme_obs.datadog.DatadogTrace).
• A shared require_extra(...) for uniform, friendly missing-dependency errors.
• A shared AKConfigError (in core/util/) replacing bare Exception.

Decisions

• Fail loud on an unknown, non-dotted-path type — drop the silent in-memory fallback (behavior change for session/thread/response stores).
• Per-surface construction contract (a BYO session store gets cache=…, a guardrail is no-arg, etc.); no forced-uniform params.
• Relax the Pydantic pattern= constraints on type fields (they currently reject dotted paths at config-load).
• Retire the Types StrEnum + from_str in the store builders.
• The pluggable ABCs (BaseTrace, InputGuardrail/OutputGuardrail, SessionStore, ThreadStore, AttachmentStore, and the sandbox ABCs) become public, stability-bearing interfaces.

Scope (incremental)

1. Shared helpers + AKConfigError, applied to trace + guardrail first.
2. Session, thread, and multimodal storage builders.
3. Converge the sandbox factory (built-ins → real imports) and the AWS response-store handler.

Design note: docs/specs/<this-issue>-pluggable-backend-factories/design.md.