feat: Add extra handlers with builder (#51)

* docs: Design for extra (non-OpenAPI) handlers + builder

* docs: Note README updates in extra-handlers design

* feat: Add ClasspathResourceHandler for static resource serving

* feat: Add Handlers.aliveHandler and Handlers.specHandler

* refactor: Use HttpURLConnection constants in alive/spec handlers

* feat: Add OpenApiServer.Builder with extra-handler support

* fix: Use responseLength=-1 for empty-body status responses

sendResponseHeaders(code, 0) triggers chunked transfer encoding for
empty bodies; -1 is correct for status-only responses with no body.
Affects notFoundHandler and the NotFound/MethodNotAllowed/default
branches of defaultExceptionHandler.

* test: Integration coverage for extra handlers and builder

* docs: Builder and extra-handlers usage in README

* style: Apply Google Java Formatter to GetDataHandler

* refactor: Hoist builder setup out of assertThatThrownBy lambdas

SonarQube java:S5778 — each throwing lambda should contain only one
invocation that could throw a runtime exception.

* test: Direct unit tests for ClasspathResourceHandler

* build: Drop deprecated <silent> parameter from dependency-plugin

maven-dependency-plugin 3.10.0 deprecates the <silent> POM parameter
in favour of the -q CLI flag; removing it silences the build warning.

* refactor: Drop redundant eq() matchers in ClasspathResourceHandlerTest

SonarQube java:S6068 — when every verify() argument is wrapped in eq(),
pass raw values instead. The eq() import is retained for the one call
that mixes eq() with longThat(), where matchers must be used uniformly.

Author: thced

README.md

@@ -83,7 +83,12 @@ public class YourServerLauncher {
     handlers.put("get-data", new GetDataHandler());
     handlers.put("post-data", new PostDataHandler());
 
-    new OpenApiServer(spec, mapper, handlers, Handlers.defaultExceptionHandler());
+    var server = OpenApiServer.builder()
+        .spec(spec)
+        .jsonMapper(mapper)
+        .handlers(handlers)
+        .exceptionHandler(Handlers.defaultExceptionHandler())
+        .build();
   }
 }
 ```
@@ -95,6 +100,35 @@ Map<String, Object> raw = new Yaml().load(Files.newInputStream(Path.of("openapi.
 ```
 The rest is identical.
 
+### Extra (non-OpenAPI) handlers
+
+Mount handlers at arbitrary paths outside the OpenAPI spec — useful for liveness probes,
+serving the spec document itself, or any other operational endpoint that should not be subject
+to OpenAPI parameter / body validation.
+
+``` java
+var server = OpenApiServer.builder()
+    .spec(spec)
+    .jsonMapper(mapper)
+    .handlers(handlers)
+    .addHandler("/alive", Handlers.aliveHandler())
+    .addHandler("/schemas/v1/openapi.yaml",
+                Handlers.specHandler("/schemas/v1/openapi.yaml"))
+    .build();
+```
+
+Extra handlers bypass OpenAPI validation but are still wrapped in the configured
+`ExceptionHandler`, so any uncaught exception is rendered using the same error envelope as
+API routes.
+
+Built-in helpers:
+- `Handlers.aliveHandler()` — 204 No Content on `GET`/`HEAD`, 405 otherwise.
+- `Handlers.specHandler(classpathResource)` — serves a classpath resource (content-type
+  inferred from extension). Throws `IllegalArgumentException` at construction if the resource
+  is missing.
+
+The original public constructors remain available for back-compat.
+
 ## Features
 - OpenAPI specification support
 - Automatic request body parsing for JSON arrays and objects