feat(params): add structured argument binding helper

Add params.Bind, which unmarshals a tool call's map[string]any args into a
typed struct via JSON round-trip so JSON numbers land in the correct Go
numeric field types, and enforces `required:"true"` struct tags with clear
errors. Migrate the branch, tree, and file repo handlers to use it instead
of repeated args["x"].(string)/!ok extraction, preserving existing
validation behavior for each field.

Co-Authored-By: Codet <codet@commitgo.dev> (GPT-5-Codex)

pkg/params/bind_test.go

@@ -0,0 +1,132 @@
+package params
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestBind_RequiredFieldsPresent(t *testing.T) {
+	type args struct {
+		Owner string `json:"owner" required:"true"`
+		Repo  string `json:"repo" required:"true"`
+	}
+	var out args
+	err := Bind(map[string]any{"owner": "gitea", "repo": "gitea-mcp"}, &out)
+	if err != nil {
+		t.Fatalf("Bind() unexpected error = %v", err)
+	}
+	if out.Owner != "gitea" || out.Repo != "gitea-mcp" {
+		t.Errorf("Bind() = %+v, want Owner=gitea Repo=gitea-mcp", out)
+	}
+}
+
+func TestBind_RequiredFieldMissing(t *testing.T) {
+	type args struct {
+		Owner string `json:"owner" required:"true"`
+	}
+	var out args
+	err := Bind(map[string]any{}, &out)
+	if err == nil {
+		t.Fatal("Bind() expected error, got nil")
+	}
+	if !strings.Contains(err.Error(), "owner") {
+		t.Errorf("Bind() error = %v, want mentioning %q", err, "owner")
+	}
+}
+
+func TestBind_RequiredStringFieldEmpty(t *testing.T) {
+	type args struct {
+		Owner string `json:"owner" required:"true"`
+	}
+	var out args
+	err := Bind(map[string]any{"owner": ""}, &out)
+	if err == nil {
+		t.Fatal("Bind() expected error for empty required string, got nil")
+	}
+}
+
+func TestBind_OptionalFieldDefaultsToZeroValue(t *testing.T) {
+	type args struct {
+		Owner     string `json:"owner" required:"true"`
+		OldBranch string `json:"old_branch"`
+	}
+	var out args
+	err := Bind(map[string]any{"owner": "gitea"}, &out)
+	if err != nil {
+		t.Fatalf("Bind() unexpected error = %v", err)
+	}
+	if out.OldBranch != "" {
+		t.Errorf("Bind() OldBranch = %q, want empty", out.OldBranch)
+	}
+}
+
+func TestBind_NumericConversion(t *testing.T) {
+	type args struct {
+		Page    int   `json:"page"`
+		PerPage int64 `json:"per_page"`
+	}
+	var out args
+	err := Bind(map[string]any{"page": float64(2), "per_page": float64(40)}, &out)
+	if err != nil {
+		t.Fatalf("Bind() unexpected error = %v", err)
+	}
+	if out.Page != 2 || out.PerPage != 40 {
+		t.Errorf("Bind() = %+v, want Page=2 PerPage=40", out)
+	}
+}
+
+func TestBind_Boolean(t *testing.T) {
+	type args struct {
+		Recursive bool `json:"recursive"`
+	}
+	var out args
+	err := Bind(map[string]any{"recursive": true}, &out)
+	if err != nil {
+		t.Fatalf("Bind() unexpected error = %v", err)
+	}
+	if !out.Recursive {
+		t.Errorf("Bind() Recursive = false, want true")
+	}
+}
+
+func TestBind_Array(t *testing.T) {
+	type args struct {
+		Labels []string `json:"labels"`
+		IDs    []int64  `json:"ids"`
+	}
+	var out args
+	err := Bind(map[string]any{
+		"labels": []any{"bug", "help wanted"},
+		"ids":    []any{float64(1), float64(2)},
+	}, &out)
+	if err != nil {
+		t.Fatalf("Bind() unexpected error = %v", err)
+	}
+	if len(out.Labels) != 2 || out.Labels[0] != "bug" || out.Labels[1] != "help wanted" {
+		t.Errorf("Bind() Labels = %v, want [bug help wanted]", out.Labels)
+	}
+	if len(out.IDs) != 2 || out.IDs[0] != 1 || out.IDs[1] != 2 {
+		t.Errorf("Bind() IDs = %v, want [1 2]", out.IDs)
+	}
+}
+
+func TestBind_InvalidFieldType(t *testing.T) {
+	type args struct {
+		Page int `json:"page"`
+	}
+	var out args
+	err := Bind(map[string]any{"page": "not-a-number"}, &out)
+	if err == nil {
+		t.Fatal("Bind() expected error for invalid numeric field, got nil")
+	}
+}
+
+func TestBind_NonPointerRejected(t *testing.T) {
+	type args struct {
+		Owner string `json:"owner"`
+	}
+	err := Bind(map[string]any{}, args{})
+	if err == nil {
+		t.Fatal("Bind() expected error for non-pointer out, got nil")
+	}
+}