Fix four issues flagged by copilot-pull-request-reviewer on PR #2143:
1. Probe URL missing /v1 prefix (openai_apikey_responses_probe.go)
Replaced bare TrimSuffix + "/responses" with buildOpenAIResponsesURL(),
which handles bare domain → /v1/responses correctly. Affected:
- ProbeOpenAIAPIKeyResponsesSupport (probe URL)
- TestAccount endpoint (apiURL for APIKey accounts)
2. Create endpoint not triggering probe (account_handler.go)
Capture created account from idempotent closure and call
scheduleOpenAIResponsesProbe after success, same pattern as
BatchCreate and Update.
3. Tests (openai_gateway_chat_completions_raw_test.go)
Added TestBuildOpenAIChatCompletionsURL (7 cases covering
bare domain, /v1 suffix, trailing slash, third-party domains,
whitespace) and TestBuildOpenAIResponsesURL_ProbeURL (6 cases
locking the probe URL construction for bare-domain inputs).
All unit tests pass; go build ./cmd/server/ clean.
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