fix streaming accept header

[sarptandoven]

summary

• set the default Accept header to text/event-stream when request options enable streaming
• keep application/json for non-streaming requests
• preserve caller-provided Accept headers for custom streaming transports

why

• closes Inquiry about the 'Accept' header #375
• streaming responses are returned as text/event-stream, and some OpenAI-compatible gateways validate the request Accept header before returning an SSE response
• callers can already work around this with a manual header override; this makes the default client behavior match the expected response type

validation

• pnpm exec jest tests/index.test.ts -t 'accept header' --runInBand
• pnpm exec jest tests/index.test.ts --runInBand
• pnpm exec tsc --noEmit --pretty false
• git diff --check
• pnpm lint
• pnpm test