Hardens session-affinity key extraction: no more JSON.stringify on arbitrary request objects,
recognizes bounded text from Responses/chat/Anthropic/Gemini/common string-root shapes, enforces
a shared 4096-char processing budget, and rejects oversized explicit session IDs before
trim/regex/hash work. 128/128 focused affinity/failover tests passing. Closes#11744. Thanks!
