fix: change SSE keepalive from event to comment format

[danielzhou82]

Summary

• Change SSE keepalive ping from event: ping\ndata: {"type": "ping"}\n\n (SSE event) to : keepalive\n\n (SSE comment) across all streaming handlers
• SSE comments keep the HTTP connection alive (preventing proxy idle timeouts) but are not yielded by the Anthropic SDK to consumers, so client-side idle watchdogs (e.g. Claude Code's 90s timeout) continue counting down correctly
• This fixes a bug where Claude Code hangs indefinitely during extended thinking: the upstream connection dies silently, sub2api's keepalive pings reset the client watchdog, and the client never retries

Problem

When using Claude Code through sub2api with extended thinking enabled, the client occasionally hangs (time keeps ticking but no tokens arrive). Root cause:

1. Upstream connection (sub2api → Anthropic) dies during long thinking pauses
2. sub2api's bufio.Scanner.Scan() blocks forever (Go's http.Transport has no read timeout on response body)
3. sub2api sends keepalive pings as SSE events → Anthropic SDK yields them → Claude Code's idle watchdog resets
4. Result: client never detects the stall, hangs indefinitely

Fix

Replace SSE event pings with SSE comments in all 4 streaming paths:

• gateway_service.go — main Claude streaming handler
• antigravity_gateway_service.go — two Antigravity streaming handlers
• openai_gateway_messages.go — OpenAI-compat Anthropic streaming handler
• gateway_helper.go — SSEPingFormatClaude constant

SSE comments (:<space>text\n\n) are valid SSE frames that keep HTTP connections alive but are silently discarded by spec-compliant SSE parsers, including the Anthropic SDK.

Test plan

• Verify streaming responses work normally (no regression in keepalive behavior)
• Test extended thinking scenario through a proxy with idle timeout < keepalive interval
• Confirm Claude Code's watchdog triggers correctly when upstream connection dies

🤖 Generated with Claude Code