feat(repository): add composite primary key support

[cofin]

Summary

This PR adds support for composite (multi-column) primary keys throughout the repository and service layers. Now you can work with association tables, legacy databases with natural keys, or any model that uses multiple columns as its primary key.

What's Changed

Repository Layer

• New PrimaryKeyType type alias that accepts scalar values, tuples, or dicts
• Helper methods for composite key handling (_build_pk_filter(), _extract_pk_value(), etc.)
• get(), delete(), and delete_many() now work with composite keys
• Uses tuple_().in_() for efficient bulk operations
• PK columns are cached in __init__ for better performance

MSSQL Compatibility

• MSSQL doesn't support row value comparisons like WHERE (col1, col2) IN ((v1, v2), ...)
• Added a dialect-specific fallback that uses OR/AND conditions instead
• Automatically detected and applied when using MSSQL

Service Layer

• get(), delete(), and delete_many() signatures updated to accept composite keys
• Added docstrings with usage examples

Memory Repository

• Full composite PK support for testing
• New helper properties and methods for key handling

Documentation

• Added "Composite Primary Keys" section to repositories docs with examples
• Added brief composite PK section to services docs with cross-reference

CI Fix

• Added sphinx to mypy's skip-imports list to fix Python 3.12+ syntax errors

Usage

# Get by composite key (tuple format - values in PK column order)
assignment = await repo.get((user_id, role_id))

# Get by composite key (dict format - explicit column names)
assignment = await repo.get({"user_id": 1, "role_id": 5})

# Bulk delete with composite keys
await repo.delete_many([
    (1, 5),
    (1, 6),
    (2, 5),
])

# Services work the same way
item = await service.get((tenant_id, item_id))
await service.delete({"tenant_id": 1, "item_id": 42})

Follow-up

• Update update() and upsert() methods for composite key support (Phase 3.3)

Closes #189

[codecov-commenter]

Codecov Report

❌ Patch coverage is 52.58964% with 119 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.70%. Comparing base (e306121) to head (8485c21).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
advanced_alchemy/repository/_sync.py	39.78%	43 Missing and 13 partials ⚠️
advanced_alchemy/repository/_async.py	79.56%	12 Missing and 7 partials ⚠️
advanced_alchemy/repository/memory/_async.py	32.14%	19 Missing ⚠️
advanced_alchemy/repository/memory/_sync.py	32.14%	19 Missing ⚠️
advanced_alchemy/repository/memory/base.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #640      +/-   ##
==========================================
- Coverage   81.17%   80.70%   -0.47%     
==========================================
  Files          99       99              
  Lines        7990     8159     +169     
  Branches     1079     1116      +37     
==========================================
+ Hits         6486     6585      +99     
- Misses       1180     1244      +64     
- Partials      324      330       +6     

☔ 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.