feat(llm-routing): accept and persist routing expressions in the function API

[along-2017]

Description

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

Let function owners persist routing expressions in llmConfig.routingMethod.
The function management API validates syntax only, against the same RFC 8941
profile the router parses: a well-formed expression persists even when its
method, parameter, type, or range is semantically wrong. The router is the
semantic authority and rejects such values at request time.

The value is stored as received apart from leading and trailing spaces: no
lowercasing, underscore folding, or alias mapping. The router already folds
case and underscores and resolves alias spellings when it resolves the
algorithm name, so every spelling accepted today keeps working. Tabs, line
breaks, and other control characters anywhere in the value are rejected.
Expressions keep the existing per-function scope: an update on any version
propagates to all versions.

There is no feature flag and no deploy ordering. In an environment whose
routers do not parse expressions yet, a model configured with parameters is
rejected by the router at request time until the routers upgrade or the owner
reverts to a method-only value.

Definition of Done

• Well-formed expressions persist via create, version create, and update
  in JSON model definitions; malformed expressions are rejected with a
  message naming the model and the violated rule
• Semantically wrong but well-formed expressions persist by design
• The persisted value equals the submitted value apart from leading and
  trailing spaces
• Updates propagate across the function's versions as today
• Table-driven tests cover the grammar profile, the stored form, and both
  API paths

Resources

• Parent: feat(llm-routing): support safe, tunable per-model routing configuration #536
• Implementation: feat(cloud-functions): accept routing expressions in routingMethod #1955
• Blocked by: none