test(coderd/x/chatd/chattest): table the OpenAI fake positive tests and fail on error streams - #30948

The six OpenAI fake positive tests in openai_test.go become three table tests, TestOpenAI_StreamingText, TestOpenAI_NonStreamingText and TestOpenAI_ToolCalls, each with a Completions API row and a Responses API row. The rows also check more than the old tests did: streaming rows fail on any error part and assert the exact text deltas, and non-streaming rows assert the returned text. The old Responses streaming test passed on a stream that held only an error, and both non-streaming tests checked only NoError and NotNil.

Why it is safe

• Every old assertion maps to a row: the exact deltas, one tool execution, the follow-up request after it, NoError and NotNil.
• I ran 12 mutants of the fake with go test -overlay. Seven pass the old test that targets the mutated behavior and fail the new rows: an error-only Responses stream, an error part after the Completions text, an error instead of response.completed, empty non-streaming text in either format, and doubled or dropped Responses deltas. The other five (an error-only Completions stream, doubled Completions deltas, a truncated OpenAITextChunks, and a dropped tool call in either format) fail both the old tests and the new rows.
• The fake is unchanged; the stronger checks pass on main.

Review focus

The Responses streaming row now uses the same two-choice text chunks as the Completions row, so both formats share one expected delta list.

Validation

• go test ./coderd/x/chatd/chattest/ -count=1: ok
• 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 (this PR)
2. test(coderd/x/chatd/chattest): pin the fake response-shape errors #30949 pin the fake response-shape errors
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.