fix(ai): send responses instructions at top level - #45085

Summary

• lower canonical initial Responses instructions into the standard top-level instructions field instead of a synthetic system input message
• remove the redundant Responses provider-option instruction path so request.system is the single source of truth
• preserve chronological developer messages and HTTP/WebSocket continuation behavior
• verify Codex OAuth routing, bearer authentication, account headers, and joined initial instructions in Core
• update affected OpenAI/xAI recorded request fixtures without changing provider response recordings

Verification

• bun test test/provider/openai-responses.test.ts test/provider/openai-compatible-responses.test.ts test/provider/xai-responses.test.ts test/provider/openai-responses-websocket.recorded.test.ts test/provider/openai-responses-cache.recorded.test.ts --timeout 30000 --only-failures from packages/ai
• RECORDED_PROVIDER=openai bun test test/provider/golden.recorded.test.ts --timeout 30000 --only-failures from packages/ai
• RECORDED_PROVIDER=openai,xai bun test test/provider/pdf.recorded.test.ts --timeout 30000 --only-failures from packages/ai
• bun test test/aisdk-native.test.ts test/model-resolver.test.ts test/plugin/provider-openai.test.ts --timeout 30000 --only-failures from packages/core
• bun typecheck from packages/ai and packages/core