feat(llm-routing): parse routing expressions and apply tuning overlays in the router

Description

Part of #536 (per-model routing configuration).

Teach the request router to accept a routing method expression, an algorithm
name plus scalar tuning parameters, in the routing-method header. Parse it as a
profile of RFC 8941 Item with Parameters. Resolve the algorithm through the
existing configured/default/request-algorithm chain, clone the resolved base
configuration, and overlay the supplied parameters; omitted fields keep their
static values. Reject a semantically invalid expression (unknown method or
parameter, incompatible parameter, invalid value, inert combination) with HTTP
400 before provider selection, using the existing rejection logging shape.

Method-only values keep today's behavior. This ticket builds definitions per
request without identity reuse; production enablement waits for the cache
ticket.

Definition of Done

• Well-formed expressions for every supported algorithm parse and route
  with the requested overlays applied; omitted fields keep static values
• Method-only values behave exactly as today
• Semantically invalid expressions return 400 before provider selection
  with a logged reason
• Every direct scalar load-balancer configuration field is supported and
  its algorithm applicability enumerated
• Table-driven tests cover grammar acceptance, rejection, and overlay
  semantics

Resources

• Parent: #536
• Blocked by: none