fix(coderd): accept RFC 9728 resource metadata with array-valued resource (#28592)

GitLab's official MCP server returns `"resource"` as a JSON array in its
`/.well-known/oauth-protected-resource` document, while RFC 9728 §2
defines it as a string. Coder's OAuth2 auto-discovery failed to decode
the response (`json: cannot unmarshal array into Go struct field
protectedResourceMetadata.resource of type string`), blocking
zero-config `oauth2` DCR against GitLab.

This makes `protectedResourceMetadata.Resource` tolerate both a single
string and an array of strings.

Part of
[CODAGT-570](https://linear.app/codercom/issue/CODAGT-570/coder-agents-mcp-client-cannot-connect-to-gitlabs-official-mcp-server).
The other blocker in that issue (GitLab replying 204 to
`notifications/initialized`) was already fixed by the migration from
`mark3labs/mcp-go` to `modelcontextprotocol/go-sdk`, whose
streamable-HTTP client accepts 204.

<details>
<summary>Investigation notes</summary>

CODAGT-570 reported two independent incompatibilities with GitLab CE's
`/api/v4/mcp` server:

1. **204 vs 202 on `notifications/initialized` (blocked all auth
modes):** the bundled `mark3labs/mcp-go` client only accepted 200/202
for notification POSTs. That library has since been removed entirely;
`coderd/x/chatd/mcpclient` now uses
`github.com/modelcontextprotocol/go-sdk v1.7.0`, which accepts both 204
and 202 (and only warns on other codes in non-strict mode, which is the
only mode reachable from Coder). No further change needed.
2. **RFC 9728 `resource` array (blocked zero-config oauth2 DCR):**
Coder's own parser in `coderd/mcp.go` declared `Resource string`. This
lives outside the MCP library, so the SDK migration did not fix it.
Fixed here with a custom `UnmarshalJSON` accepting string or array of
strings. The field is only decoded, never consumed downstream, so
behavior is otherwise unchanged.

</details>

---

Generated by Coder Agents on behalf of @f0ssel.

coderd/mcp.go

@@ -1699,9 +1699,29 @@ type mcpOAuth2Discovery struct {
 // protectedResourceMetadata represents the response from a
 // Protected Resource Metadata endpoint per RFC 9728 §2.
 type protectedResourceMetadata struct {
-	Resource             string   `json:"resource"`
-	AuthorizationServers []string `json:"authorization_servers"`
-	ScopesSupported      []string `json:"scopes_supported,omitempty"`
+	Resource             resourceIdentifiers `json:"resource"`
+	AuthorizationServers []string            `json:"authorization_servers"`
+	ScopesSupported      []string            `json:"scopes_supported,omitempty"`
+}
+
+// resourceIdentifiers tolerates both a single JSON string and an
+// array of strings. RFC 9728 §2 defines "resource" as a string, but
+// some servers (e.g. GitLab's official MCP server) return an array
+// when the metadata document covers multiple resources.
+type resourceIdentifiers []string
+
+func (r *resourceIdentifiers) UnmarshalJSON(data []byte) error {
+	var single string
+	if err := json.Unmarshal(data, &single); err == nil {
+		*r = resourceIdentifiers{single}
+		return nil
+	}
+	var many []string
+	if err := json.Unmarshal(data, &many); err != nil {
+		return xerrors.New("resource must be a string or an array of strings")
+	}
+	*r = resourceIdentifiers(many)
+	return nil
 }
 
 // authServerMetadata represents the response from an Authorization

coderd/mcp_internal_test.go

@@ -2,6 +2,7 @@ package coderd
 
 import (
 	"context"
+	"encoding/json"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -214,3 +215,55 @@ func TestOIDCMCPTokenSource(t *testing.T) {
 		require.Empty(t, tok)
 	})
 }
+
+func TestProtectedResourceMetadataResourceUnmarshal(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		input   string
+		want    resourceIdentifiers
+		wantErr bool
+	}{
+		{
+			name:  "String",
+			input: `{"resource": "https://gitlab.example.com/api/v4/mcp", "authorization_servers": ["https://gitlab.example.com"]}`,
+			want:  resourceIdentifiers{"https://gitlab.example.com/api/v4/mcp"},
+		},
+		{
+			// GitLab's official MCP server returns an array of
+			// resources despite RFC 9728 defining a string.
+			name:  "Array",
+			input: `{"resource": ["https://gitlab.example.com/api/v4/mcp", "https://gitlab.example.com/api/v4/orbit/mcp"], "authorization_servers": ["https://gitlab.example.com"]}`,
+			want: resourceIdentifiers{
+				"https://gitlab.example.com/api/v4/mcp",
+				"https://gitlab.example.com/api/v4/orbit/mcp",
+			},
+		},
+		{
+			name:  "Absent",
+			input: `{"authorization_servers": ["https://gitlab.example.com"]}`,
+			want:  nil,
+		},
+		{
+			name:    "InvalidType",
+			input:   `{"resource": 42, "authorization_servers": ["https://gitlab.example.com"]}`,
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			var meta protectedResourceMetadata
+			err := json.Unmarshal([]byte(tt.input), &meta)
+			if tt.wantErr {
+				require.Error(t, err)
+				return
+			}
+			require.NoError(t, err)
+			require.Equal(t, tt.want, meta.Resource)
+		})
+	}
+}