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_error_compliance_test.go

@@ -356,11 +356,14 @@ func TestOAuth2ErrorHTTPHeaders(t *testing.T) {
 func TestOAuth2SpecificErrorScenarios(t *testing.T) {
 	t.Parallel()
 
+	// Single instance shared across all sub-tests that need a
+	// coderd server. Sub-tests that don't need one just ignore it.
+	client := coderdtest.New(t, nil)
+	_ = coderdtest.CreateFirstUser(t, client)
+
 	t.Run("MissingRequiredFields", func(t *testing.T) {
 		t.Parallel()
 
-		client := coderdtest.New(t, nil)
-		_ = coderdtest.CreateFirstUser(t, client)
 		ctx := testutil.Context(t, testutil.WaitLong)
 
 		// Test completely empty request
@@ -385,8 +388,6 @@ func TestOAuth2SpecificErrorScenarios(t *testing.T) {
 	t.Run("UnsupportedFields", func(t *testing.T) {
 		t.Parallel()
 
-		client := coderdtest.New(t, nil)
-		_ = coderdtest.CreateFirstUser(t, client)
 		ctx := testutil.Context(t, testutil.WaitLong)
 
 		// Test with fields that might not be supported yet
@@ -408,8 +409,6 @@ func TestOAuth2SpecificErrorScenarios(t *testing.T) {
 	t.Run("SecurityBoundaryErrors", func(t *testing.T) {
 		t.Parallel()
 
-		client := coderdtest.New(t, nil)
-		_ = coderdtest.CreateFirstUser(t, client)
 		ctx := testutil.Context(t, testutil.WaitLong)
 
 		// Register a client first