feat(nodes): add pluggable routing pipeline

[localai-org-maint-bot]

Summary

• route loaded-replica selection through composable candidate filters, named weighted scorers, and a replaceable picker
• preserve the existing load guard, hot-prefix, cold-placement, least-loaded fallback, and deterministic replica tie-break behavior
• expose validated per-model scorer_weights through scheduling YAML/JSON, REST persistence, and the database model
• document the pipeline and its initial prefix_cache scorer

Closes #10066.

Verification

• go test -count=1 ./core/services/nodes/prefixcache
• go test -count=1 ./core/services/nodes -ginkgo.focus=ParseSchedulingSeed
• go test -run "^$" ./core/services/nodes/... ./core/http/endpoints/localai
• go vet ./core/services/nodes/prefixcache ./core/services/nodes
• git diff HEAD^ --check

The full nodes integration suite was also attempted: 254 specs passed, while 200 database-backed specs failed at setup because this host has no rootless Docker provider/PostgreSQL test container.

Checklist

• Tests added for pipeline filtering, weighted-sum scoring, abstaining signals, deterministic/default and custom pickers, scorer validation, and declarative config parsing
• User-facing distributed-mode documentation updated
• No coverage baseline or tolerance changes