test(coderd/x/chatd): cover BYOK header choice for Bedrock and OpenAI-compatible routes - #30978

TestResolveModelRouteForConfigAIGatewayProviderAuth gains one subtest that pins which header carries a user's own provider key on two more routes: X-Api-Key for an Anthropic model on a Bedrock provider, and Authorization: Bearer for an OpenAI-compatible provider. The next layer rewrites how chatd picks that header, and until now no test failed when the call site dropped the model ID. This test-only layer is the characterization test for that rewrite, so it adds lines instead of removing them.

Why it is needed

• resolveAIGatewayRoute passes the model ID to aiGatewayRequestFormatForProviderType, and only the Bedrock arm reads it. A mutant that passes "" instead passed all of go test ./coderd/x/chatd/: no chatd test resolved a BYOK route for a Bedrock provider, and TestAIGatewayRequestFormatForProviderType calls the classifier directly.
• With this subtest, that mutant fails on the Bedrock row (X-Api-Key expected, Authorization sent). A mutant that maps the classifier's default arm to the Anthropic format fails on the OpenAI-compatible row. A garbage copy of model_routing_aibridge.go fails to build, so the -overlay applies.

Review focus

Today the OpenAI-compatible row takes the same default arm as the OpenAI UserKey subtest. The next layer keys the header on the client family (anthropic, openai, openai-compat), and this row fails if that switch matches only openai.

Validation

• go test ./coderd/x/chatd/ -run '^TestResolveModelRouteForConfigAIGatewayProviderAuth$' -count=1: pass.
• Both mutants above, through go test -overlay: fail the new subtest.
• go test ./coderd/x/chatd/ -count=1 -p 4 -parallel 4: 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; #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.