fix(tools,gitea): alias repo/index args and reject empty path segments

Per-repo MCP tools 404'd while owner-level tools worked (#36). Root cause was
a parameter-name contract mismatch, not a routing fault: every per-repo tool
declares the repo identifier as 'name' (and issue/PR index as 'number'), but
every caller — the claude.ai connector, LLMs primed on gitea's own API — sends
'repo' and 'index'. The unmatched fields zero-valued the upstream path segment,
producing '/api/v1/repos/{owner}//...', which gitea answers with its generic
api-404 whose body points at /api/swagger. That swagger pointer is gitea boiler-
plate, not a misroute — the MCP dispatch was correct all along.

Two layers of defence:
- parseArgs aliases repo->name and index->number (explicit canonical wins;
  alias key left intact so pr_merge's real 'index' field is unaffected). Kills
  the recurrence by accepting the idiomatic argument names.
- the gitea client rejects any path with an empty segment before the HTTP call,
  returning ErrValidation instead of forwarding a malformed path and surfacing
  gitea's opaque swagger-404. Kills the silent-misleading-error class.

Closes #36

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

internal/gitea/client_guard_test.go

@@ -0,0 +1,54 @@
+package gitea_test
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"sync/atomic"
+	"testing"
+
+	"gitea.d-ma.be/mathias/gitea-mcp/internal/gitea"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// #36 second line of defence: when owner or repo is empty the path contains an
+// empty segment ("//"). Rather than forward it upstream — where gitea answers
+// with its opaque /api/swagger 404 — the client must reject it locally with a
+// validation error and never touch the network.
+func TestEmptyPathSegmentRejectedBeforeNetwork(t *testing.T) {
+	var hits int32
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		atomic.AddInt32(&hits, 1)
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+
+	paths := []string{
+		"/api/v1/repos/mathias//issues",        // empty repo
+		"/api/v1/repos//gitea-mcp/contents/",    // empty owner
+		"/api/v1/repos/mathias//issues?state=open", // empty repo before query
+	}
+	for _, p := range paths {
+		_, _, err := c.GetJSON(context.Background(), p)
+		require.Error(t, err, "path %q should be rejected", p)
+		assert.ErrorIs(t, err, gitea.ErrValidation)
+	}
+	assert.Equal(t, int32(0), atomic.LoadInt32(&hits), "guard must short-circuit before any HTTP call")
+}
+
+// A well-formed path with a query string must still pass the guard.
+func TestWellFormedPathPasses(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`[]`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	_, status, err := c.GetJSON(context.Background(), "/api/v1/repos/mathias/gitea-mcp/issues?state=open")
+	require.NoError(t, err)
+	assert.Equal(t, 200, status)
+}