Drop WebContents → BindCallback registry on BravePassageEmbeddingsService
There was ever at most one active embedder/BG WebContents pair, so the
static map was 0- or 1-entry overhead. Replace it with a direct
forwarder:
* BravePassageEmbeddingsService::BindLocalAIReceiver hands a
PendingReceiver<LocalAIService> to batch_embedder_ if alive.
* BravePassageEmbeddingsServiceController exposes the same as an
instance method, accessible via Get().
* UntrustedLocalAIUI::BindInterface routes through the controller.
Eliminates SetBindCallbackForWebContents (only producer call site went
away) and RemoveBindCallbackForWebContents (was unused in production —
the registry leaked one entry per BG WebContents lifecycle).
Anthony Tseng
d0cebb06d7