fix(routing): auto-migrate v0.3.0 inline routing_preferences to v0.4.0 top-level (#912)

* fix(routing): auto-migrate v0.3.0 inline routing_preferences to v0.4.0 top-level

Lift inline routing_preferences under each model_provider into the
top-level routing_preferences list with merged models[] and bump
version to v0.4.0, with a deprecation warning. Existing v0.3.0
demo configs (Claude Code, Codex, preference_based_routing, etc.)
keep working unchanged. Schema flags the inline shape as deprecated
but still accepts it. Docs and skills updated to canonical top-level
multi-model form.

* test(common): bump reference config assertion to v0.4.0

The rendered reference config was bumped to v0.4.0 when its inline
routing_preferences were lifted to the top level; align the
configuration deserialization test with that change.

* fix(config_generator): bump version to v0.4.0 up front in migration

Move the v0.3.0 -> v0.4.0 version bump to the top of
migrate_inline_routing_preferences so it runs unconditionally,
including for configs that already declare top-level
routing_preferences at v0.3.0. Previously the bump only fired
when inline migration produced entries, leaving top-level v0.3.0
configs rejected by brightstaff's v0.4.0 gate. Tests updated to
cover the new behavior and to confirm we never downgrade newer
versions.

* fix(config_generator): gate routing_preferences migration on version < v0.4.0

Short-circuit the migration when the config already declares v0.4.0
or newer. Anything at v0.4.0+ is assumed to be on the canonical
top-level shape and is passed through untouched, including stray
inline preferences (which are the author's bug to fix). Only v0.3.0
and older configs are rewritten and bumped.

demos/llm_routing/claude_code_router/config.yaml

@@ -19,7 +19,7 @@ model_providers:
       - name: code understanding
         description: understand and explain existing code snippets, functions, or libraries
   # Anthropic Models
-  - model: anthropic/claude-sonnet-4-5
+  - model: anthropic/claude-sonnet-4-6
     default: true
     access_key: $ANTHROPIC_API_KEY