fix(samples): exit early when DI app key is still the placeholder

The library's ValidateOnStart only rejects a blank ApiKey, so the shipped
"REPLACE_ME" placeholder passed validation and the app started with an
invalid key — failing only at the first API call. Detect the placeholder
explicitly and print the same configuration guidance, matching the spec's
"missing or placeholder -> exit non-zero" intent. README updated to match.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: HandyS11

samples/README.md

@@ -30,6 +30,9 @@ dotnet user-secrets set "RustMaps:ApiKey" "YOUR_KEY" \
 dotnet run --project samples/RustMapsApi.DependencyInjection.ConsoleApp
 ```
 
+Until a real key is supplied, the app detects the `REPLACE_ME` placeholder (and a
+blank key via `ValidateOnStart`), prints the command above, and exits non-zero.
+
 ## Menu
 
 ```text

samples/RustMapsApi.DependencyInjection.ConsoleApp/Program.cs

@@ -18,6 +18,15 @@
 // User-secrets overrides the appsettings.json placeholder in Development.
 builder.Configuration.AddUserSecrets(typeof(Program).Assembly, optional: true);
 
+// The library only rejects a blank key, so the shipped "REPLACE_ME" placeholder would
+// otherwise start the app with an invalid key and fail at the first API call. Catch it here.
+const string PlaceholderApiKey = "REPLACE_ME";
+if (builder.Configuration["RustMaps:ApiKey"] == PlaceholderApiKey)
+{
+    await WriteConfigurationHelpAsync("The RustMaps API key is still the placeholder.");
+    return 1;
+}
+
 builder.Services.AddRustMapsClientV4(builder.Configuration.GetSection("RustMaps"));
 
 try
@@ -32,9 +41,14 @@
 }
 catch (OptionsValidationException ex)
 {
-    await Console.Error.WriteLineAsync($"Configuration error: {string.Join("; ", ex.Failures)}");
+    await WriteConfigurationHelpAsync($"Configuration error: {string.Join("; ", ex.Failures)}");
+    return 1;
+}
+
+static async Task WriteConfigurationHelpAsync(string reason)
+{
+    await Console.Error.WriteLineAsync(reason);
     await Console.Error.WriteLineAsync(
         "Set the key with: dotnet user-secrets set \"RustMaps:ApiKey\" \"YOUR_KEY\" " +
         "--project samples/RustMapsApi.DependencyInjection.ConsoleApp");
-    return 1;
 }