media_chain: buildDefaultChain uses hardcoded provider names instead of registry lookup by type

Problem

buildDefaultChain in internal/tools/media_provider_chain.go iterates over a hardcoded priority list of provider names (e.g. "openrouter", "gemini", "anthropic", "dashscope") and calls registry.Get(ctx, name) to check availability.

This breaks when providers are registered under different names (e.g. aistudio-google instead of gemini, or alibabacloud instead of dashscope). The chain comes back empty even though a compatible provider is available.

Additionally, ResolveMediaProviderChain returns the builtin_tools.settings chain immediately without checking if those providers are actually registered, causing a silent "all providers failed" error at call time.

Root cause

// buildDefaultChain — looks up by hardcoded name, not by type
for _, name := range priority {
    if _, err := registry.Get(ctx, name); err == nil { // fails if registered as "aistudio-google"
        ...
    }
}

// ResolveMediaProviderChain — returns settings chain without registry validation
chain := parseChainSettings(raw, defaultModels)
if len(chain) > 0 {
    return chain // may contain providers not in registry
}

Suggested fix

1. buildDefaultChain: iterate registry.List(ctx) instead of the hardcoded names. Resolve each provider's type via ResolveProviderType(p), then match against defaultModels (keyed by type). Sort results by the priority type list.

2. ResolveMediaProviderChain: after parsing the settings chain, filter entries through registry.Get(). If the filtered result is empty, log a warning and fall through to buildDefaultChain.

// buildDefaultChain — iterate all registered providers, filter by type capability
byType := make(map[string][]MediaProviderEntry)
for _, name := range registry.List(ctx) {
    p, _ := registry.Get(ctx, name)
    pType := ResolveProviderType(p)
    if model, ok := defaultModels[pType]; ok {
        byType[pType] = append(byType[pType], MediaProviderEntry{Provider: name, Model: model, Enabled: true})
    }
}
// order by priority type list
for _, pType := range priority {
    chain = append(chain, byType[pType]...)
}

// ResolveMediaProviderChain — filter settings chain by registry
var available []MediaProviderEntry
for _, e := range chain {
    if _, err := registry.Get(ctx, e.Provider); err == nil {
        available = append(available, e)
    } else {
        slog.Warn("media_chain: configured provider not registered, skipping", "tool", toolName, "provider", e.Provider)
    }
}
if len(available) > 0 {
    return available
}
slog.Warn("media_chain: all configured providers unavailable, falling back to defaults", "tool", toolName)
// fall through to buildDefaultChain

Impact

Affects all media tools that use ResolveMediaProviderChain: read_image, read_video, read_audio, read_document, create_image, create_audio, create_video.

Start in internal/tools/media_provider_chain.go by tracing buildDefaultChain and ResolveMediaProviderChain through registry.List, registry.Get, ResolveProviderType, and the defaultModels priority list. Verify behavior for providers registered under aliases and for unavailable configured entries across the listed media tools; done means compatible registered providers are selected and settings fall back when none are available.