Fix OpenAI messages empty stream handling

[stgmt]

Summary

Fix the OpenAI /v1/messages Anthropic-compat streaming bridge so pre-output upstream failures are not surfaced to Claude-compatible clients as successful empty streams or generic 502s.

This patch:

• pre-estimates input tokens from the converted Responses request and uses that value in Anthropic message_start.usage.input_tokens
• buffers initial Anthropic SSE events until real visible assistant output or tool output starts
• treats terminal OpenAI Responses streams with no visible assistant/tool output as failover errors before any client bytes are written
• maps upstream context_length_exceeded / no-visible-output client failures to non-retryable Anthropic-compatible 4xx errors instead of masking them as Upstream service temporarily unavailable
• lets the messages failover loop retry only when no model output has actually reached the client
• adds diagnostics for terminal Responses event types, output item types, usage, and context-window errors

Why

Claude-compatible clients can treat an Anthropic stream that only contains message_start / message_stop as a completed assistant turn even when OpenAI Responses actually ended with response.failed, response.incomplete, or reasoning-only output. In large-context sessions this hides actionable upstream errors such as context_length_exceeded behind an empty reply or a generic 502.

Tests

• go test ./internal/pkg/apicompat -run TestStreamingMessageStartUsesPrefilledInputTokens -count=1
• go test ./internal/service -run 'TestOpenAIMessagesAnthropicStream(ContextLengthBeforeVisibleOutputReturnsClientError|BuffersStartUntilVisibleOutput)' -count=1
• go test ./internal/handler -run TestOpenAIHandleAnthropicFailoverExhaustedPreservesClientError -count=1

Notes

I did not run the full unit/integration/lint matrix locally; the change is covered by targeted regression tests around the affected bridge path.

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[stgmt]

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