refactor(coderd/x/chatd): derive the BYOK auth header from the AI Gateway client choice - #30979

fantasyConfigForAIBridge picked the fantasy client for an AI Gateway route, and aiGatewayRequestFormatForProviderType mapped the same provider type and model again, only to choose X-Api-Key or Authorization: Bearer for a user's own key. This PR moves the client choice into aibridgeClientProvider and passes its result to aiGatewayProviderAuthForUser, so the header follows the client. It deletes the request-format enum, the mapper, the mapper's test, and the superseded TestNewModelAttribution.

Why it is safe

• aibridgeClientProvider is the old fantasyConfigForAIBridge switch without the base URL. TestAIBridgeProviderFormatMapping (12 rows, unchanged) still pins the client and base URL; the mapper test's 9 rows are a subset of them.
• The mapper returned the Anthropic format for exactly the inputs that get the Anthropic client, so no header changes. TestAIGatewayProviderAuthForUser and the test(coderd/x/chatd): cover BYOK header choice for Bedrock and OpenAI-compatible routes #30978 subtest pin the header.
• TestNewModelAttributionImmutablePerRequest covers both workspace bindings that TestNewModelAttribution checked, plus the API key ID, also without a database.
• Mutants through go test -overlay each fail a remaining test: every arm of aibridgeClientProvider and the base URL check; an auth switch keyed on openai, an auth case matching openai-compat instead of anthropic, and a dropped model ID at the resolveAIGatewayRoute call; the workspace check, the workspace ID copy, and an empty API key ID. A garbage copy fails to build, so the overlay applies.
• rg -n 'aiGatewayRequestFormat' . finds nothing.

Review focus

Only the Anthropic client gets X-Api-Key; openai and openai-compat keep Authorization: Bearer, as the old default format gave them. Only the #30978 subtest catches an auth switch keyed on openai.

Validation

• go test ./coderd/x/chatd/... -count=1 -p 4 -parallel 4: pass.
• 12 mutants: each fails a remaining test.
• 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; #30979 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. refactor(coderd/x/chatd): derive the BYOK auth header from the AI Gateway client choice #30979 Derive the BYOK auth header from the client choice

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