refactor: centralize optional-dependency typing shims

[cofin]

Summary

Centralizes every optional-dependency feature flag and stub type into a single canonical module, and enforces that internal code only imports from the public surface.

Stacked on top of #716 — diff will collapse to a single commit once that merges.

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 to import from advanced_alchemy.typing instead of reaching into private _typing modules.

Test plan

• make lint passes
• uv run pytest tests/unit -x -q passes
• Public re-exports unchanged (from advanced_alchemy.service import ATTRS_INSTALLED still works)
• CI green

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.55823% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.22%. Comparing base (8d61227) to head (f34718e).

Files with missing lines	Patch %	Lines
advanced_alchemy/utils/serialization.py	86.59%	22 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #717    +/-   ##
========================================
  Coverage   81.21%   81.22%            
========================================
  Files          99      100     +1     
  Lines        8300     8576   +276     
  Branches     1124     1160    +36     
========================================
+ Hits         6741     6966   +225     
- Misses       1232     1271    +39     
- Partials      327      339    +12     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.