Bump semantic-kernel from 1.35.3 to 1.39.3 in /src/backend

Bumps [semantic-kernel](https://github.com/microsoft/semantic-kernel) from 1.35.3 to 1.39.3.
- [Release notes](https://github.com/microsoft/semantic-kernel/releases)
- [Commits](microsoft/semantic-kernel@python-1.35.3...python-1.39.3)

---
updated-dependencies:
- dependency-name: semantic-kernel
  dependency-version: 1.39.3
  dependency-type: direct:production
...

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

Author: dependabot[bot]

src/backend/pyproject.toml

@@ -27,7 +27,7 @@ dependencies = [
     "pytest-cov==5.0.0",
     "python-dotenv==1.1.1",
     "python-multipart==0.0.20",
-    "semantic-kernel==1.35.3",
+    "semantic-kernel==1.39.3",
     "uvicorn==0.35.0",
     "pylint-pydantic==0.3.5",
     "pexpect==4.9.0",