fix(tools): add actionable guidance when validate-urls is missing httpx

[shuofengzhang]

What changed

• Added a dependency guard in tools/validate-urls.py that catches ModuleNotFoundError for httpx.
• Replaced the raw traceback with an actionable message telling contributors to run the script with uv run (or install httpx).
• Returned exit code 2 for this missing-dependency case so CI/local tooling can detect setup issues clearly.

Why

• Running python3 tools/validate-urls.py in an environment without httpx currently fails with a Python traceback.
• This small change improves contributor UX by surfacing a clear, direct remediation path instead of an internal stack trace.
• Behavior is unchanged when httpx is installed.

Testing

• scripts/clone_and_test.sh awslabs/agent-plugins
  • Tests: not run (no tests found)
• python3 tools/validate-urls.py
  • Verified graceful missing-dependency message and exit code 2 when httpx is not installed.

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.