chore(deps): bump ruff from 0.15.20 to 0.15.21 in /claude-agent-sdk

Bumps [ruff](https://github.com/astral-sh/ruff) from 0.15.20 to 0.15.21.
- [Release notes](https://github.com/astral-sh/ruff/releases)
- [Changelog](https://github.com/astral-sh/ruff/blob/main/CHANGELOG.md)
- [Commits](astral-sh/ruff@0.15.20...0.15.21)

---
updated-dependencies:
- dependency-name: ruff
  dependency-version: 0.15.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

claude-agent-sdk/pyproject.toml

@@ -18,7 +18,7 @@ test = [
 ]
 dev = [
     "bolt-python-support-agent-claude[test]",
-    "ruff==0.15.20",
+    "ruff==0.15.21",
 ]
 
 [tool.setuptools.packages.find]

claude-agent-sdk/requirements.txt

@@ -7,4 +7,4 @@ python-dotenv==1.2.2
 
 pytest==9.1.1
 pytest-asyncio==1.4.0
-ruff==0.15.20
+ruff==0.15.21