fix(proxy): keep SpendLogs and callback session ids in sync when the request has none (#39450)

* fix(proxy): keep SpendLogs and callback session ids in sync when the request has none

Add general_settings.missing_session_id (generate | reject). In generate mode one id is
stamped into litellm_session_id, litellm_trace_id and metadata.session_id before callbacks
run, so LiteLLM_SpendLogs.session_id and the Langfuse session id match. In reject mode such
requests get a 400. Unset keeps the legacy behavior. MCP routes are not affected

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* chore(proxy): regenerate schema.d.ts and shorten mutable-ok comment for ruff format

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* fix(proxy): mark generated session ids so affinity consumers do not pin on them

Fireworks x-session-affinity, the router session_affinity pre-call check and the
complexity router session pin all read metadata.session_id as a caller-chosen
stable key. A missing_session_id: generate id is fresh per request, so it now
carries metadata.litellm_session_id_generated and those consumers skip it

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: yassin <yassin@berri.ai>
Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>