refactor: centralize optional-dependency typing shims (#719)

## Summary

Centralizes every optional-dependency feature flag and stub type into a single canonical module, and stops internal code from reaching into private `_typing` modules.

## Changes

- Make `advanced_alchemy/_typing.py` the single source of truth for all `*_INSTALLED` flags and optional-dependency stub types (pydantic, msgspec, attrs, cattrs, litestar, sqlmodel, numpy, orjson).
- Re-export the full surface from the public `advanced_alchemy/typing.py`.
- Delete `advanced_alchemy/service/_typing.py` — its contents were generic shims with nothing service-specific.
- Update internal callers (`service/__init__.py`, `service/_util.py`, `service/typing.py`, `repository/_typing.py`) to import from `advanced_alchemy.typing` instead of reaching into private `_typing` modules.

Author: cofin

.pre-commit-config.yaml

@@ -22,7 +22,7 @@ repos:
       - id: unasyncd
         additional_dependencies: ["ruff"]
   - repo: https://github.com/charliermarsh/ruff-pre-commit
-    rev: "v0.15.9"
+    rev: "v0.15.12"
     hooks:
       # Run the linter.
       - id: ruff