[follow-up] audio / images / completions handlers dispatch via Provider enum, not Hub

Follow-up to api7/AISIX-Cloud#417 close (api7/ai-gateway#365) — surfaced by audit MEDIUM-3.

Gap

The chat / chat_stream / embed / messages / responses path goes through Hub::dispatch_two_tier and benefits from the new Adapter::Openai family bridge fallthrough. But three other proxy handlers still go through the legacy require_provider(model) -> Provider flow + provider.default_base_url():

• crates/aisix-proxy/src/audio.rs:295,418
• crates/aisix-proxy/src/images.rs
• crates/aisix-proxy/src/completions.rs

Even with Provider::Xai now in the enum (so the chat path works), a Model loaded with a brand-new long-tail vendor (not yet added to the enum but admitted by cp-api via provider_metadata) would:

• chat → routes via Adapter::Openai family bridge → works ✅
• audio/images/completions → require_provider returns an unknown variant → schema validation actually catches this first (model_schema's closed enum), so the symptom is "Model row never loads" rather than a 5xx at request time

But once #302 Phase A opens the model_schema provider field to free-form string (the cleanup item I listed in api7/AISIX-Cloud#302), audio/images/completions handlers will start returning enum-mismatch errors that the chat path no longer does.

Fix sketch

Refactor require_provider / resolve_base_url to also consult ProviderKey.api_base first and fall back to the legacy Provider::default_base_url() only when populated. Or stop dispatching off Model.provider for audio/images/completions and route those through the Hub family bridge too (parity with chat).

Why not in #365

• The chat path is the customer-blocking surface from #417; audio/images is not exercised by the #417 repro
• The fix is non-trivial (different handler structure than chat) and the right pattern depends on Phase A's broader refactor direction
• Documented here so it doesn't get lost when #302 Phase A flips model_schema provider to free-form

Start with require_provider and resolve_base_url, then read the handler paths in crates/aisix-proxy/src/audio.rs at lines 295 and 418, crates/aisix-proxy/src/images.rs, and crates/aisix-proxy/src/completions.rs. Compare their dispatch with the Hub::dispatch_two_tier chat path and review ProviderKey.api_base and Provider::default_base_url. Done means these handlers remain compatible with free-form providers after model_schema Phase A without losing legacy fallback behavior.