fix(gemini): filter noisy stderr output from gemini backend (#83)

* fix(gemini): filter noisy stderr output from gemini backend

- Add filteringWriter to filter [STARTUP], Warning, Session cleanup etc.
- Apply filter only for gemini backend stderr output
- Add unit tests for filtering logic

* fix: use defer for stderrFilter.Flush to cover all return paths

Address review feedback: ensure filter is flushed on failure paths

codeagent-wrapper/filter.go

@@ -0,0 +1,66 @@
+package main
+
+import (
+	"bytes"
+	"io"
+	"strings"
+)
+
+// geminiNoisePatterns contains stderr patterns to filter for gemini backend
+var geminiNoisePatterns = []string{
+	"[STARTUP]",
+	"Session cleanup disabled",
+	"Warning:",
+	"(node:",
+	"(Use `node --trace-warnings",
+	"Loaded cached credentials",
+	"Loading extension:",
+	"YOLO mode is enabled",
+}
+
+// filteringWriter wraps an io.Writer and filters out lines matching patterns
+type filteringWriter struct {
+	w        io.Writer
+	patterns []string
+	buf      bytes.Buffer
+}
+
+func newFilteringWriter(w io.Writer, patterns []string) *filteringWriter {
+	return &filteringWriter{w: w, patterns: patterns}
+}
+
+func (f *filteringWriter) Write(p []byte) (n int, err error) {
+	f.buf.Write(p)
+	for {
+		line, err := f.buf.ReadString('\n')
+		if err != nil {
+			// incomplete line, put it back
+			f.buf.WriteString(line)
+			break
+		}
+		if !f.shouldFilter(line) {
+			f.w.Write([]byte(line))
+		}
+	}
+	return len(p), nil
+}
+
+func (f *filteringWriter) shouldFilter(line string) bool {
+	for _, pattern := range f.patterns {
+		if strings.Contains(line, pattern) {
+			return true
+		}
+	}
+	return false
+}
+
+// Flush writes any remaining buffered content
+func (f *filteringWriter) Flush() {
+	if f.buf.Len() > 0 {
+		remaining := f.buf.String()
+		if !f.shouldFilter(remaining) {
+			f.w.Write([]byte(remaining))
+		}
+		f.buf.Reset()
+	}
+}