refactor(coderd/x/chatd): drop the remaining direct provider clients - #30929

After #30928, chatprovider.ModelFromConfig still built direct Azure, Google, OpenRouter, and Vercel clients. The AI Gateway path never asks for them, because fantasyConfigForAIBridge sends those provider types through the OpenAI-compatible client. This PR deletes those four arms and their missing-key error cases, the per-provider region plumbing that only the deleted Bedrock client read, and ProviderAPIKeys.Empty, which has no callers left.

Why it is safe

• The only production caller of ModelFromConfig passes a hint from fantasyConfigForAIBridge (see refactor(coderd/x/chatd/chatprovider): drop the direct Bedrock client #30928), and no test builds these four clients.
• Region flowed from ConfiguredProvider.Region through ResolveUserProviderKeys into ProviderAPIKeys.RegionByProvider, and only the Bedrock arm read it. The gateway reads Bedrock and Claude Platform regions from the provider settings itself (aibridgedserver.go).
• chatd.go still marks Claude Platform on AWS as supporting ambient credentials. TestResolveQuickgenModel_TitleGenerationOverrideAmbientCredentials fails when that flag is forced to false.
• Empty lost its last production caller when direct chat routing was removed (refactor: deprecate AIGatewayRoutingEnabled, remove direct chat routing #26862). Its only test checked the region term.
• Deleted tests (TestProviderAPIKeysEmpty_RegionCountsAsConfigured, TestResolveUserProviderKeys_PreservesProviderRegions, the DisabledProviderRegionsRemoved prune row) checked region and Empty code that is gone. One also checked that a keyless Bedrock provider is available; TestResolveUserProviderKeys still fails when the Bedrock ambient case or the empty-key rule in setResolvedProviderAPIKey is mutated.
• rg -n 'RegionByProvider|setRegion|keys\.Region\(|\.Empty\(\)' coderd/x/chatd finds nothing.

Review focus

The aiProviderConfigFromKeys hunk in chatd.go, the only change outside chatprovider: it drops the region reads and keeps the ambient credential flag.

Validation

• go test ./coderd/x/chatd/chatprovider/... ./coderd/x/chatd/chatopenai/...: pass.
• go test ./coderd/x/chatd/ -count=1 -p 4 -parallel 4: pass.
• go build ./... and go vet on every package that imports chatprovider: pass.
• make pre-commit: pass.

Stack

This stack removes provider code that only direct, non-gateway clients used, so chatprovider builds just the three clients the AI Gateway path needs, then derives the BYOK auth header from that same client choice.

1. refactor(coderd/x/chatd/chatprovider): drop the direct Bedrock client #30928 Drop the direct Bedrock client
2. refactor(coderd/x/chatd): drop the remaining direct provider clients #30929 Drop the remaining direct provider clients
3. refactor(coderd/x/chatd/chatprovider): drop provider options only direct clients read #30958 Drop provider options only direct clients read
4. refactor(coderd/x/chatd): drop transport branches only direct clients used #30966 Drop transport branches only direct clients used
5. test(coderd/x/chatd): cover BYOK header choice for Bedrock and OpenAI-compatible routes #30978 Cover the BYOK header choice for Bedrock and OpenAI-compatible routes
6. Share the AI Gateway client choice with BYOK auth (next)

Xum, an AI agent, opened this PR for @ibetitsmike and wrote this description.