feat(registry): model metadata capture and resolution

Config gets two new surfaces (llmgateway.go): Config.ModelMeta
(map[string]*ModelMetaConfig, keyed by exact "provider/model" or a
bare model id) for operator overrides, and ProviderConfig.MetadataPath
for openai-type providers whose discovery should also capture per-model
context (LM Studio's native /api/v0/models shape).

modelmeta.go adds the layering itself: resolveModelMeta applies config
override > discovered context > builtin table > absent for context, and
config override/free > builtin > absent for cost (discovery never
reports cost). resolveAliasModelMeta implements alias inheritance: an
alias inherits its target's fully resolved metadata unless the alias
itself has its own modelMeta entry, applied per field. validateModelMeta
rejects negative values and a free:true/non-zero-cost contradiction at
construction.

registry.go wires this into the request path: providerState gains a
discoveredContext map (stale-while-error, mirroring discovered's own
convention) with contextFor/setDiscoveredContext; a new optional
modelMetadataFetcher interface, type-asserted (interpreted-to-interpreted,
documented as yaegi-safe against the Trap-4 restriction) rather than
added to providerAdapter, lets warmFill/refreshProvider capture metadata
alongside listModels without touching anthropic/gemini adapters at all.
modelObject/listFor now attach resolved context_window/pricing to every
GET /v1/models entry, including aliases (resolveMetaFor/
resolveMetaForAlias/resolveMetaForAliasName).

provider_openai.go implements modelMetadataFetcher: fetchModelMetadata
reads ProviderConfig.MetadataPath, preferring loaded_context_length over
max_context_length when the LM Studio response reports both. providers.go
wires MetadataPath onto the adapter in buildAdapters.

tools/yaegi-check/main.go adds one cheap modelMeta config-override
assertion to the existing attempt-accounting harness, proving
resolveModelMeta's config-override layer and the context_window/pricing
extension fields both run correctly under the real Yaegi interpreter.

providers.go

@@ -417,6 +417,7 @@ func buildAdapters(cfg *Config) (map[string]providerAdapter, error) {
 		case providerTypeOpenAI:
 			a := newOpenAIAdapter(name, base, apiKey)
 			a.retry = policy
+			a.metadataPath = pc.MetadataPath
 			adapters[name] = a
 		case providerTypeAnthropic:
 			a, err := newAnthropicAdapter(name, base, apiKey)