[dev-loop] Add tests for fetchFileContext, fetchPatterns, and persona edge cases

- Add mock vcsClient for unit testing helper functions in cmd/review-bot
- Add 11 tests for fetchFileContext: empty files, removed file skip, content
  fetching, error continuation, context cancellation
- Add 6 tests for fetchPatterns: empty repo, all files, specific files,
  invalid repo format, fetch errors, multiple repos
- Add 4 tests for review/persona: LoadPersona nonexistent/non-regular/oversized,
  CapitalizeFirst RuneError path

Coverage: cmd/review-bot 37.6% → 46.1%, review 91.5% → 92.0%

review/persona_test.go

@@ -957,3 +957,51 @@ func TestYAMLMergeKeyDepthCheck(t *testing.T) {
 		t.Errorf("error = %q, want to contain 'depth'", err.Error())
 	}
 }
+
+func TestLoadPersona_NonexistentFile(t *testing.T) {
+	_, err := LoadPersona("/tmp/nonexistent-persona-file-xyz.yaml")
+	if err == nil {
+		t.Fatal("expected error for nonexistent file, got nil")
+	}
+}
+
+func TestLoadPersona_NotARegularFile(t *testing.T) {
+	// Use a directory as the path — directories are not regular files.
+	dir := t.TempDir()
+	_, err := LoadPersona(dir)
+	if err == nil {
+		t.Fatal("expected error for directory path, got nil")
+	}
+	if !strings.Contains(err.Error(), "not a regular file") {
+		t.Errorf("error = %q, want to contain 'not a regular file'", err.Error())
+	}
+}
+
+func TestLoadPersona_OversizedFile(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "big.yaml")
+	// Write a file larger than MaxPersonaFileSize
+	data := make([]byte, MaxPersonaFileSize+1)
+	for i := range data {
+		data[i] = 'x'
+	}
+	if err := os.WriteFile(path, data, 0644); err != nil {
+		t.Fatalf("failed to create test file: %v", err)
+	}
+	_, err := LoadPersona(path)
+	if err == nil {
+		t.Fatal("expected error for oversized file, got nil")
+	}
+	if !strings.Contains(err.Error(), "exceeds maximum size") {
+		t.Errorf("error = %q, want to contain 'exceeds maximum size'", err.Error())
+	}
+}
+
+func TestCapitalizeFirst_RuneError(t *testing.T) {
+	// An invalid UTF-8 byte sequence should return the original string unchanged.
+	invalid := string([]byte{0xFF, 0xFE})
+	got := CapitalizeFirst(invalid)
+	if got != invalid {
+		t.Errorf("CapitalizeFirst(%q) = %q, want original %q", invalid, got, invalid)
+	}
+}