Policies backend (#6527)

# Description of Changes
Add a backend for running any multi-step PDF operations. This is
designed to be used for the upcoming Policies feature, along with
anything else that will require automated running of PDF operations,
like the Automate tool or Processing Folders.

The implementation is not complete. I've tried to get all the
infrastructure in there so that we can add in whichever triggers we need
in the future (like cron triggers or watching folders on disk) but
currently it just supports manual triggering of the policy.

The basis of this work was the operation running from the Stirling
Engine, which this PR removes in favour of this new system. The only
currently accessible frontend way to test this work is to ask the AI
chat to execute multiple operations on a PDF, but I've also extensively
tested with direct API calls to make sure that the policies work and
persist properly.

app/common/src/main/java/stirling/software/common/service/ToolMetadataService.java

@@ -1,11 +1,21 @@
 package stirling.software.common.service;
 
+import java.util.List;
+
 /** Provides metadata about tool endpoints for internal dispatch. */
 public interface ToolMetadataService {
 
     /** Returns true if the given operation path accepts multiple input files. */
     boolean isMultiInput(String operationPath);
 
+    /**
+     * Returns the file extensions (lowercase, no leading dot, e.g. {@code "pdf"}) that the
+     * operation accepts as input ({@code output=false}) or produces as output ({@code
+     * output=true}), derived from the endpoint's declared type. Returns {@code null} when the
+     * endpoint declares no specific type, which callers should treat as "any type accepted".
+     */
+    List<String> getExtensionTypes(boolean output, String operationPath);
+
     /**
      * Returns true when the endpoint's ZIP response is a transport for multiple typed results and
      * should be unpacked: multi-output endpoints (Type:SIMO / Type:MIMO) and wrapper declarations