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/code_search.go

@@ -49,7 +49,7 @@ func (t *CodeSearch) Descriptor() registry.ToolDescriptor {
 type codeSearchArgs struct {
 	Q     string `json:"q"`
 	Owner string `json:"owner"`
-	Repo  string `json:"repo"`
+	Repo  string `json:"repo,omitempty"` // optional: empty => owner-wide fan-out (#37 opt-out)
 	Page  int    `json:"page"`
 	Limit int    `json:"limit"`
 }