feat: add read/write replica routing support

[cofin]

Summary

• Adds automatic query routing to read replicas for SELECT queries and to primary for writes
• Provides context managers (primary_context, replica_context) for explicit routing control
• Integrates with all framework extensions (Litestar, FastAPI, Starlette, Flask, Sanic)

Features

• RoutingSession / RoutingAsyncSession with automatic get_bind() routing
• RoutingConfig for configuring primary and replica connections
• Round-robin and random replica selection strategies
• Sticky-primary mode for write-then-read consistency
• FOR UPDATE query detection for automatic primary routing

Test plan

• 97 unit tests for routing components
• 21 integration tests with real SQLite databases
• All linting passes (make lint)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.26740% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.20%. Comparing base (947df81) to head (b179a91).

Files with missing lines	Patch %	Lines
advanced_alchemy/routing/session.py	97.29%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #635      +/-   ##
==========================================
+ Coverage   80.44%   81.20%   +0.76%     
==========================================
  Files          87       93       +6     
  Lines        6453     6726     +273     
  Branches      838      857      +19     
==========================================
+ Hits         5191     5462     +271     
- Misses        998      999       +1     
- Partials      264      265       +1     

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