test: reduce number of coderdtest instances (#23463)

Consolidates coderdtest invocations in 7 tests to reduce 23 instances to 7 across:
- `TestGetUser` (3 → 1) — read-only user lookups
- `TestUserTerminalFont` (3 → 1) — each creates own user via
CreateAnotherUser
- `TestUserTaskNotificationAlertDismissed` (3 → 1) — each creates own
user
- `TestUserLogin` (3 → 1) — each creates/deletes own user
- `TestExpMcpConfigureClaudeCode` (5 → 1) — writes to isolated temp dirs
- `TestOAuth2RegistrationTokenSecurity` (3 → 1) — independent
registrations
- `TestOAuth2SpecificErrorScenarios` (3 → 1) — independent error
scenarios

> 🤖 This PR was created with the help of Coder Agents, and has been
reviewed by my human. 🧑‍💻

coderd/oauth2_security_test.go

@@ -104,11 +104,14 @@ func TestOAuth2ClientIsolation(t *testing.T) {
 func TestOAuth2RegistrationTokenSecurity(t *testing.T) {
 	t.Parallel()
 
+	// Single instance shared across all sub-tests. Each registers
+	// independent OAuth2 apps with unique client names.
+	client := coderdtest.New(t, nil)
+	_ = coderdtest.CreateFirstUser(t, client)
+
 	t.Run("InvalidTokenFormats", func(t *testing.T) {
 		t.Parallel()
 
-		client := coderdtest.New(t, nil)
-		_ = coderdtest.CreateFirstUser(t, client)
 		ctx := t.Context()
 
 		// Register a client to use for testing
@@ -145,8 +148,6 @@ func TestOAuth2RegistrationTokenSecurity(t *testing.T) {
 	t.Run("TokenNotReusableAcrossClients", func(t *testing.T) {
 		t.Parallel()
 
-		client := coderdtest.New(t, nil)
-		_ = coderdtest.CreateFirstUser(t, client)
 		ctx := t.Context()
 
 		// Register first client
@@ -179,8 +180,6 @@ func TestOAuth2RegistrationTokenSecurity(t *testing.T) {
 	t.Run("TokenNotExposedInGETResponse", func(t *testing.T) {
 		t.Parallel()
 
-		client := coderdtest.New(t, nil)
-		_ = coderdtest.CreateFirstUser(t, client)
 		ctx := t.Context()
 
 		// Register a client