fix(tools): reject empty repo/name identifier at tool layer (#37)

An empty required repo identifier built a trailing-empty path segment
(`/api/v1/repos/{owner}/`) and leaked gitea's bare 404. parseArgs now
validates, via reflection, that `repo`/`name` string args are non-empty and
returns a typed ErrValidation naming the field. Optional identifiers (e.g.
code_search's owner-wide fan-out `repo`) opt out with `,omitempty`. `owner` is
already enforced by the allowlist check.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/tools/tool.go

@@ -2,8 +2,11 @@ package tools
 
 import (
 	"encoding/json"
+	"fmt"
+	"reflect"
 	"strings"
 
+	"gitea.d-ma.be/mathias/gitea-mcp/internal/gitea"
 	"gitea.d-ma.be/mathias/gitea-mcp/internal/registry"
 )
 
@@ -16,9 +19,46 @@ func textOK(v any) (json.RawMessage, error) {
 
 func parseArgs(raw json.RawMessage, dst any) error {
 	if len(raw) == 0 {
-		return json.Unmarshal([]byte("{}"), dst)
+		if err := json.Unmarshal([]byte("{}"), dst); err != nil {
+			return err
+		}
+	} else if err := json.Unmarshal(normalizeAliases(raw), dst); err != nil {
+		return err
 	}
-	return json.Unmarshal(normalizeAliases(raw), dst)
+	return validateRequiredIdentifiers(dst)
+}
+
+// validateRequiredIdentifiers rejects an empty repo identifier at the tool layer
+// before it reaches the client, where it would build a trailing-empty path
+// segment (`/api/v1/repos/{owner}/`) and leak gitea's bare 404 (#37). `repo` and
+// `name` are always required where present — the per-repo identifier tools and
+// the two create tools respectively; `owner` is already enforced by the
+// allowlist check. Returns a typed ErrValidation naming the missing field.
+func validateRequiredIdentifiers(dst any) error {
+	v := reflect.ValueOf(dst)
+	if v.Kind() != reflect.Ptr || v.IsNil() {
+		return nil
+	}
+	v = v.Elem()
+	if v.Kind() != reflect.Struct {
+		return nil
+	}
+	t := v.Type()
+	for i := 0; i < t.NumField(); i++ {
+		base, opts, _ := strings.Cut(t.Field(i).Tag.Get("json"), ",")
+		if base != "repo" && base != "name" {
+			continue
+		}
+		// An optional identifier (e.g. code_search's `repo`, empty => owner-wide
+		// fan-out) opts out by carrying `,omitempty`.
+		if strings.Contains(opts, "omitempty") {
+			continue
+		}
+		if v.Field(i).Kind() == reflect.String && v.Field(i).String() == "" {
+			return fmt.Errorf("%q is required: %w", base, gitea.ErrValidation)
+		}
+	}
+	return nil
 }
 
 // normalizeAliases reconciles the two spellings of the repo identifier so a tool