feat: add function chain

[junjiejiangjjj]

feat: decode public function chain protobuf

fix: freeze function chain representation values

fix: preserve decoded function parameter types

feat: validate and plan function chains

fix: harden function chain validation

fix: reject non-finite function chain literals

feat: support literal function chain inputs

feat: add public numeric combine expression

fix: prevent non-finite combined scores

feat: support optional function chain tie breaking

fix: make function chain sorting NaN-safe

feat: compile public function chains

feat: prepare and execute search function chains

feat: support function chains in search

test: complete function chain coverage

test: refine function chain coverage

docs: correct function chain module reference

fix: preserve function chain search compatibility

fix: harden function chain projection and validation

fix: secure rerank configuration and primary key inputs

fix: reject function chain system field conflicts

fix: reject unsupported rerank parameters

refactor: centralize rerank provider metadata

test: support eager partition key validation

docs: design L2 function chain test migration

docs: plan L2 function chain test migration

fix: reject order by with function chains

test: scaffold L2 function chain compatibility

test: harden L2 compatibility fixture cleanup

test: migrate positive L2 function chain cases

test: assert function chain fields are omitted

test: migrate L2 system column validation cases

test: migrate L2 function chain conflict cases

docs: document function chain order by conflict