feat: include platformCode in error responses (v0.10.0)

- Error(w, err) now extracts PlatformCode() from *xerrors.Err and
  includes "platformCode" in the JSON body when set; omitted otherwise
- errorBody updated to accept platformCode as an explicit parameter
- Upgraded code.nochebuena.dev/go/xerrors dependency to v0.10.0
- Added two new tests: platformCode included when set, omitted when absent

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

go.sum

@@ -2,6 +2,8 @@ code.nochebuena.dev/go/valid v0.9.0 h1:o8/tICIoed2+uwBp+TxXa3FE6KmyirU266O4jEUgF
 code.nochebuena.dev/go/valid v0.9.0/go.mod h1:SKpLcqpEsLMaEk7K3Y0kFF7Y3W5PHAQF6+U6wleFAhg=
 code.nochebuena.dev/go/xerrors v0.9.0 h1:8wrDto7e44ZW1YPOnT6JrxYXTqnvNuKpAO1/5bcT4TE=
 code.nochebuena.dev/go/xerrors v0.9.0/go.mod h1:mtXo7xscBreCB7w7smlBP5Onv8H1HVohCvF0I/VXbAY=
+code.nochebuena.dev/go/xerrors v0.10.0 h1:/4BCGZ7yZ384IC7t8SWGuAqEH/FmI9WtxiB5RKIoKWc=
+code.nochebuena.dev/go/xerrors v0.10.0/go.mod h1:mtXo7xscBreCB7w7smlBP5Onv8H1HVohCvF0I/VXbAY=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/gabriel-vasile/mimetype v1.4.12 h1:e9hWvmLYvtp846tLHam2o++qitpguFiYCKbn0w9jyqw=