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>
