fix: prevent goroutine leaks in streaming executors via context-aware channel sends

All streaming executors use bare channel sends (out <- chunk) inside goroutines
that process upstream SSE responses. When the downstream consumer disconnects
(client timeout, network drop, etc.), these sends block indefinitely, causing
the goroutine and all associated resources (HTTP response body, scanner buffers,
translation state) to leak permanently.

Over time, leaked goroutines accumulate monotonically, leading to RSS growth
from ~30MB to 3.7GB+ and eventual OOM kills on resource-constrained VPS hosts.

Fix: Replace all bare 'out <- ...' sends with:
  select {
  case out <- ...:
  case <-ctx.Done():
    return
  }

This ensures goroutines terminate promptly when the request context is canceled,
allowing GC to reclaim all associated resources.

Affected executors (9 files, 36+ send sites):
- antigravity_executor.go (5 sites)
- gemini_cli_executor.go (6 sites)
- gemini_vertex_executor.go (6 sites)
- aistudio_executor.go (4 sites)
- gemini_executor.go (3 sites)
- openai_compat_executor.go (3 sites)
- claude_executor.go (4 sites)
- codex_executor.go (2 sites)
- kimi_executor.go (3 sites)

internal/runtime/executor/gemini_executor.go

@@ -324,17 +324,28 @@ func (e *GeminiExecutor) ExecuteStream(ctx context.Context, auth *cliproxyauth.A
 			}
 			lines := sdktranslator.TranslateStream(ctx, to, from, req.Model, opts.OriginalRequest, body, bytes.Clone(payload), &param)
 			for i := range lines {
-				out <- cliproxyexecutor.StreamChunk{Payload: lines[i]}
+				select {
+				case out <- cliproxyexecutor.StreamChunk{Payload: lines[i]}:
+				case <-ctx.Done():
+					return
+				}
 			}
 		}
 		lines := sdktranslator.TranslateStream(ctx, to, from, req.Model, opts.OriginalRequest, body, []byte("[DONE]"), &param)
 		for i := range lines {
-			out <- cliproxyexecutor.StreamChunk{Payload: lines[i]}
+			select {
+			case out <- cliproxyexecutor.StreamChunk{Payload: lines[i]}:
+			case <-ctx.Done():
+				return
+			}
 		}
 		if errScan := scanner.Err(); errScan != nil {
 			helps.RecordAPIResponseError(ctx, e.cfg, errScan)
 			reporter.PublishFailure(ctx)
-			out <- cliproxyexecutor.StreamChunk{Err: errScan}
+			select {
+			case out <- cliproxyexecutor.StreamChunk{Err: errScan}:
+			case <-ctx.Done():
+			}
 		}
 	}()
 	return &cliproxyexecutor.StreamResult{Headers: httpResp.Header.Clone(), Chunks: out}, nil