test(coderd/x/chatd/chattest): pin the fake response-shape errors - #30949

No test reached the "handler returned both streaming and non-streaming responses" and "handler returned empty response" branches of the chattest fakes, and the next PR in the stack rewrites them. This PR adds TestFakeResponseShapeErrors, which posts to the OpenAI /chat/completions, OpenAI /responses and Anthropic /v1/messages fakes with a handler that returns both response shapes or neither, and checks the 500 status and the exact error body. It is the test-first layer for that rewrite, so it adds 77 lines and deletes none.

Why it is safe

• Test only, and it passes on main.
• I deleted each of the six cases in turn with go test -overlay. Each mutant fails exactly its own row, and the rest of the package still passes on all six, so no other test covered these branches.

Review focus

The test sends plain net/http requests with a {} body, so it asserts the fake's own output (http.Error writes the message and a newline) with no provider SDK in between.

Validation

• go test ./coderd/x/chatd/chattest/ -run TestFakeResponseShapeErrors -count=1 -v: all six rows pass
• go vet ./coderd/x/chatd/chattest/: ok
• golangci-lint run --fix=false ./coderd/x/chatd/chattest/: 0 issues
• pre-commit hook: passed

Stack

The stack makes the chattest fake tests fail on wrong output, then drops the fakes' write-only state and shares their response-shape check.

1. test(coderd/x/chatd/chattest): table the OpenAI fake positive tests and fail on error streams #30948 table the OpenAI fake positive tests and fail on error streams
2. test(coderd/x/chatd/chattest): pin the fake response-shape errors #30949 pin the fake response-shape errors (this PR)
3. refactor(coderd/x/chatd/chattest): drop write-only fake state and share the shape check #30950 drop write-only fake state and share the shape check

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