fix: use plugin_dirs config to resolve external plugins (#2935)

[omorros]

🔗 Related Issue
Closes #2935

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📝 Summary

The plugin_dirs config field in plugins/config.yaml was never used by the plugin
manager. External plugins in custom directories failed to load because their paths were
not added to sys.path before importlib.import_module() was called.

This PR adds the configured plugin_dirs to sys.path during PluginManager.initialize()
and cleans them up on shutdown() and reset().

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🏷️ Type of Change

• Bug fix

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🧪 Verification

Command: pytest tests/unit/mcpgateway/plugins/framework/test_manager.py -v
Status: 13/13 passed

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✅ Checklist

• Code formatted (make black isort pre-commit)
• Tests added/updated for changes
• Documentation updated (if applicable)
• No secrets or credentials committed

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📓 Notes (optional)

• Only mcpgateway/plugins/framework/manager.py was changed (production code)
• Directories are only added if they exist on disk (os.path.isdir check)
• sys.path is cleaned up in both shutdown() and reset() to avoid leaking paths across
  tests or restarts

[omorros]

All Python tests and coverage checks pass on both py3.11 and py3.12. The only failing check is linting-full, which seems to be a Helm unittest plugin installation issue unrelated to this PR. Let me know if anything needs to be changed! ;)