fix: prevent empty/failed streaming responses in OpenCode proxy

[BP602]

Why

OpenCode requests were intermittently failing with connection reset by peer or returning empty answers. Logs showed two concrete causes:

• Bun server idle timeout closed active SSE responses, then stream writes crashed against a closed controller
• stream event filtering could incorrectly drop text events, producing empty output even when upstream completed

What changed

• hardened SSE enqueue/close handling to avoid closed-controller cascades
• added configurable CLAUDE_PROXY_IDLE_TIMEOUT_SECONDS to support long-lived streams
• fixed stream event forwarding behavior so text deltas are not incorrectly skipped
• split debug levels so baseline diagnostics stay useful while noisy stream internals are optional

Result

• streaming responses stay open long enough to complete
• no closed-controller error loops after disconnects/timeouts
• OpenCode receives non-empty responses consistently in tested scenarios