fix(apicompat): support array-format content in streaming chat completions

[YanzheL]

Summary

• normalize text-only messages[].content arrays into a plain string in the chat-completions → responses compatibility path
• preserve multimodal arrays as structured content so image inputs continue to pass through correctly
• add regression coverage for both the compat converter and the streaming OpenAI chat-completions forwarding path

Problem

• OpenAI-compatible clients such as Cursor can send chat-completions messages[].content as an array instead of a string
• streaming /v1/chat/completions requests could fail on that payload shape and return 502 upstream_error
• plain string content and non-streaming requests were not affected

Fix

• flatten pure text content arrays into a single string during ChatCompletionsToResponses
• keep mixed or multimodal arrays in structured Responses API part form instead of collapsing them
• add a service-level streaming regression test that verifies the forwarded upstream payload for Cursor-style requests

Verification

• go test ./internal/pkg/apicompat
• go test -tags unit ./internal/service -run 'TestExtractCCReasoningEffortFromBody|TestHandleCCBufferedFromAnthropic_PreservesMessageStartCacheUsageAndReasoning|TestHandleCCStreamingFromAnthropic_PreservesMessageStartCacheUsageAndReasoning|TestOpenAIGatewayService_ForwardAsChatCompletions_StreamTextArrayContent'
• go test ./...
• go build ./cmd/server
• go test -tags=unit ./...
• go test -tags=integration ./...
• golangci-lint run ./... (not available in this environment: command not found)

Closes #1247