fix(pipeline): allow slashes in pipeline operation values (#4066)

# Description of Changes

- Extended the validation regex for `operation` in the pipeline
processor to allow slashes (`/`), in addition to alphanumeric
characters, underscores, and hyphens.
- This resolves the issue where valid operation strings (e.g., with
subpaths like `/api/v1/general/remove-pages`) were incorrectly rejected.
- Added an explicit log message for better debugging in case of invalid
`operation` values.

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

app/core/src/test/java/stirling/software/SPDF/controller/api/pipeline/PipelineProcessorTest.java

@@ -45,23 +45,26 @@ class PipelineProcessorTest {
     @Test
     void runPipelineWithFilterSetsFlag() throws Exception {
         PipelineOperation op = new PipelineOperation();
-        op.setOperation("filter-page-count");
+        op.setOperation("/api/v1/filter/filter-page-count");
         op.setParameters(Map.of());
         PipelineConfig config = new PipelineConfig();
         config.setOperations(List.of(op));
 
-        Resource file = new ByteArrayResource("data".getBytes()) {
-            @Override
-            public String getFilename() {
-                return "test.pdf";
-            }
-        };
+        Resource file =
+                new ByteArrayResource("data".getBytes()) {
+                    @Override
+                    public String getFilename() {
+                        return "test.pdf";
+                    }
+                };
 
         List<Resource> files = List.of(file);
 
-        when(apiDocService.isMultiInput("filter-page-count")).thenReturn(false);
-        when(apiDocService.getExtensionTypes(false, "filter-page-count"))
+        when(apiDocService.isMultiInput("/api/v1/filter/filter-page-count")).thenReturn(false);
+        when(apiDocService.getExtensionTypes(false, "/api/v1/filter/filter-page-count"))
                 .thenReturn(List.of("pdf"));
+        when(apiDocService.isValidOperation(eq("/api/v1/filter/filter-page-count"), anyMap()))
+                .thenReturn(true);
 
         doReturn(new ResponseEntity<>(new byte[0], HttpStatus.OK))
                 .when(pipelineProcessor)