docs(auto_routing): session_affinity is on by default

[devin-ai-integration]

Summary

complexity_router now defaults session_affinity to on (BerriAI/litellm#33500), so the auto_routing docs are updated to match. The "Session affinity" section now reads "On by default", documents session_affinity: false as the opt-out, and notes that pinning a session to one model also avoids provider errors when conversation history from one model (for example an Anthropic thinking block) is replayed to a different model on a later turn. The config snippet comment now reads session_affinity: true # default; set false to reclassify every turn.

Link to Devin session: https://app.devin.ai/sessions/fcac1b22aa084efa9f36dc8f99976601
Requested by: @krrish-berri-2

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Project	Deployment	Actions	Updated (UTC)
litellm	Ready	Preview, Comment	Jul 16, 2026 5:05am