Generate the OpenAPI spec on a free port (#7777)

`generateOpenApiDocs` forks a Spring Boot instance to harvest the spec,
but the scrape URL was hardcoded to `http://localhost:8080` - the app's
own port. If any Stirling instance is already listening there (a dev
server, a container, a WSL relay) the plugin reads **that** build
instead of the fork, and `task tool-models` then generates the Python
and TypeScript models from whatever happens to be running.

It fails silently and looks like success. I hit it while regenerating
models after editing Java annotations: the build passed, the compiled
class contained the new text, and the spec contained none of it.
Fetching the URL by hand settled it:

```
paths live8080: 276   ours: 276
identical parsed content: True
```

The generated `SwaggerDoc.json` was the running instance's document, not
the working tree's.

Now on a random free port, passed to both the fork and the scrape URL so
they cannot disagree. Override with `-PopenApiPort=NNNN` to use a
specific port.

Author: Frooodle

build.gradle

@@ -452,8 +452,12 @@ subprojects {
     if (name == 'stirling-pdf') {
         apply plugin: 'org.springdoc.openapi-gradle-plugin'
 
+        def openApiPort = rootProject.findProperty('openApiPort')?.toString()
+            ?: new ServerSocket(0).withCloseable { it.localPort }.toString()
+
         openApi {
-            apiDocsUrl = "http://localhost:8080/v1/api-docs"
+            apiDocsUrl = "http://localhost:${openApiPort}/v1/api-docs"
+            customBootRun { args = ["--server.port=${openApiPort}"] }
             outputDir = file("$projectDir")
             outputFileName = "SwaggerDoc.json"
             waitTimeInSeconds = 60 // Increase the wait time to 60 seconds