fix: correct plugin catalog path resolution in is_valid_plugin (fixes #245)

[Flamki]

Fixes #245.

Description

is_valid_plugin() was resolving plugin_names.json using os.path.abspath(__file__) as if it were a directory, which can produce an invalid path and raise FileNotFoundError.

This PR fixes path resolution and adds a regression unit test.

Changes

• chatbot-core/api/tools/utils.py
  • resolve path from module directory:
    • tools_dir = os.path.dirname(os.path.abspath(__file__))
    • os.path.normpath(os.path.join(tools_dir, "..", "..", "data", "raw", "plugin_names.json"))
• chatbot-core/tests/unit/tools/test_utils.py
  • test_is_valid_plugin_returns_true_for_known_plugin
  • test_is_valid_plugin_returns_false_for_unknown_plugin

Why this is aligned / scoped

• Small backend reliability fix (no architecture change)
• Directly improves plugin retrieval stability
• No overlap with auth/design-phase Java/Gatekeeper work

Testing done

• pytest tests/unit/tools/test_utils.py -q -> 2 passed
• pylint api/tools/utils.py tests/unit/tools/test_utils.py -> 10.00/10

Submitter checklist

• Topic branch used
• PR title reflects intended changelog
• Changes described
• Linked issue
• Added tests for the fix