exit: cap initial downstream response size

[poulcarlsen53]

Summary

• cap the first downstream response for a newly opened session to 512 KiB by default
• keep first responses urgent so setup bytes are returned without coalesce delay
• let later responses use the normal large response budget for bulk throughput
• add optional initial_response_bytes_pre_encode server config wiring for operators who want to tune the cap

Why

Apps Script buffers the full HTTP response before the client can decode the first byte. A huge first response can make downloads look like they take many seconds to start even when steady-state throughput is high. This keeps the first visible response small while preserving later bulk batches.

Verification

• go test -count=1 ./internal/exit -run TestDrainAll_CapsInitialResponseOnly
• go test -count=1 ./internal/config -run TestLoadServerInitialResponseBytesPreEncode
• go test -count=1 ./...
• go vet ./...