fix(coderd/x/chatd): show correct provider and clean detail for Bedrock errors (#26338)

## Problem
Bedrock errors (routed through aibridge) showed the wrong provider
("Anthropic ...")
and a doubly-wrapped detail string instead of the clean message.

## Fix (chatd only)
- **Provider label:** thread the configured provider to error
classification via
`GenerateAssistantOptions.ErrorProvider`. Transport provider still
drives prompt
  prep, sanitization, and metric labels (unchanged).
- **Detail:** unwrap the SDK transport wrapper (`METHOD "URL": NNN
{body}`) to surface
the inner message; handles top-level `{"message":...}` and nested
`{"error":{"message":...}}`.

## Notes
- Surfacing a top-level `message` now applies to all providers
(intentional; nested wins when both present).
- The advisor path keeps the transport label; accepted as-is (it returns
`err.Error()`, not the classification).
- Fixes display in chatd only; the wrapper originates in aibridge (out
of scope here).

🤖 Generated by Coder Agents.

coderd/x/chatd/generation.go

@@ -48,6 +48,10 @@ type generationPrepared struct {
 	ModelRoute        resolvedModelRoute
 	ModelBuildOptions modelBuildOptions
 
+	// ResolvedProvider is the configured provider identity used to label
+	// user-facing errors. See chatloop.GenerateAssistantOptions.ErrorProvider.
+	ResolvedProvider string
+
 	ModelConfigID        uuid.UUID
 	ModelConfig          codersdk.ChatModelCallConfig
 	ProviderOptions      fantasy.ProviderOptions
@@ -626,6 +630,7 @@ func (s *taskStarter) generateAssistant(
 	runCtx := input.DebugTurn.Ensure(ctx, prepared.Chat, prepared.Debug)
 	outcome, err := chatloop.GenerateAssistant(runCtx, chatloop.GenerateAssistantOptions{
 		Model:                prepared.Model,
+		ErrorProvider:        prepared.ResolvedProvider,
 		Messages:             prepared.Prompt,
 		Tools:                prepared.Tools,
 		ActiveTools:          prepared.ActiveTools,