Bump sentence-transformers from 5.2.2 to 5.5.1 in /backend-agent

Bumps [sentence-transformers](https://github.com/huggingface/sentence-transformers) from 5.2.2 to 5.5.1.
- [Release notes](https://github.com/huggingface/sentence-transformers/releases)
- [Commits](huggingface/sentence-transformers@v5.2.2...v5.5.1)

---
updated-dependencies:
- dependency-name: sentence-transformers
  dependency-version: 5.5.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

backend-agent/pyproject.toml

@@ -35,7 +35,7 @@ dependencies = [
     'codeattack @ git+https://github.com/marcorosa/CodeAttack',
     'gptfuzzer @ git+https://github.com/marcorosa/GPTFuzz@no-vllm',
     'garak==0.11.0',
-    'sentence-transformers==5.2.2',
+    'sentence-transformers==5.5.1',
 ]
 
 [project.optional-dependencies]