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_test.go

@@ -0,0 +1,73 @@
+package main
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestFilteringWriter(t *testing.T) {
+	tests := []struct {
+		name     string
+		patterns []string
+		input    string
+		want     string
+	}{
+		{
+			name:     "filter STARTUP lines",
+			patterns: geminiNoisePatterns,
+			input:    "[STARTUP] Recording metric\nHello World\n[STARTUP] Another line\n",
+			want:     "Hello World\n",
+		},
+		{
+			name:     "filter Warning lines",
+			patterns: geminiNoisePatterns,
+			input:    "Warning: something bad\nActual output\n",
+			want:     "Actual output\n",
+		},
+		{
+			name:     "filter multiple patterns",
+			patterns: geminiNoisePatterns,
+			input:    "YOLO mode is enabled\nSession cleanup disabled\nReal content\nLoading extension: foo\n",
+			want:     "Real content\n",
+		},
+		{
+			name:     "no filtering needed",
+			patterns: geminiNoisePatterns,
+			input:    "Line 1\nLine 2\nLine 3\n",
+			want:     "Line 1\nLine 2\nLine 3\n",
+		},
+		{
+			name:     "empty input",
+			patterns: geminiNoisePatterns,
+			input:    "",
+			want:     "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var buf bytes.Buffer
+			fw := newFilteringWriter(&buf, tt.patterns)
+			fw.Write([]byte(tt.input))
+			fw.Flush()
+
+			if got := buf.String(); got != tt.want {
+				t.Errorf("got %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestFilteringWriterPartialLines(t *testing.T) {
+	var buf bytes.Buffer
+	fw := newFilteringWriter(&buf, geminiNoisePatterns)
+
+	// Write partial line
+	fw.Write([]byte("Hello "))
+	fw.Write([]byte("World\n"))
+	fw.Flush()
+
+	if got := buf.String(); got != "Hello World\n" {
+		t.Errorf("got %q, want %q", got, "Hello World\n")
+	}
+}