Fix path injection, sensitive attribute exposure, and error body truncation

Security fixes from audit:

- Escape user-supplied strings (ext_relation_id, interface_name) with
  url.PathEscape before interpolating into API URL paths, preventing
  path traversal via crafted values like "../admin" or "foo/bar"
- Mark auth token URL attributes as Sensitive in both
  virtfusion_user_auth_token and virtfusion_user_server_auth_token
  resources, since the URL embeds the signed token
- Truncate raw API error response bodies to 500 bytes in error messages
  to prevent leaking sensitive data from verbose Laravel error responses

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/provider/data_source_user.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"net/url"
 
 	"terraform-provider-virtfusion/internal/client"
 
@@ -105,7 +106,7 @@ func (d *UserDataSource) Read(ctx context.Context, req datasource.ReadRequest, r
 		return
 	}
 
-	rawResp, err := d.client.Get(ctx, fmt.Sprintf("/users/%s/byExtRelation", data.ExtRelationID.ValueString()))
+	rawResp, err := d.client.Get(ctx, fmt.Sprintf("/users/%s/byExtRelation", url.PathEscape(data.ExtRelationID.ValueString())))
 	if err != nil {
 		resp.Diagnostics.AddError("Error Reading User", err.Error())
 		return