fix(apicompat): harden Responses and Anthropic tool bridging

[laozuzhen]

Summary

• Harden Responses API <-> Anthropic Messages conversion for polymorphic input items, tool-call pairing, empty tool inputs, and text/tool output deltas.
• Add output validation for Anthropic HTTP 200 responses so empty/error/zero-usage bodies fail over instead of being recorded as successful completions.
• Preserve Anthropic message cache-control transforms and request-body context when forwarding Responses requests upstream.

Tests

• go test ./internal/pkg/apicompat ./internal/service -run 'Responses|Anthropic|ChatCompletions|Tool|Empty|ForwardAsResponses|ApiKeyPassthrough|MessageCache'

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