chore(examples): move 43_mixed_marketplace_skills to 05_skills_and_plugins

[Uszkido]

HUMAN:
Moving the mixed marketplace skills example to the dedicated skills and plugins directory for better organization, as outlined in issue #2496.

AGENT:

Why

The 01_standalone_sdk/ directory had grown to 44+ sequentially numbered examples. A dedicated 05_skills_and_plugins/ category was created for plugin/skill examples, and this example belongs there.

Summary

• Moved examples/01_standalone_sdk/43_mixed_marketplace_skills/ to examples/05_skills_and_plugins/05_mixed_marketplace_skills/
• Updated path references in 03_managing_installed_skills/main.py
• Updated path references in 05_mixed_marketplace_skills/main.py and README.md
• Updated test path in tests/examples/test_examples.py
• Left gap in 01_standalone_sdk/ numbering to keep the PR minimal

How to Test

1. Verify the folder exists at examples/05_skills_and_plugins/05_mixed_marketplace_skills/
2. Verify the old path examples/01_standalone_sdk/43_mixed_marketplace_skills/ no longer exists
3. Run the examples tests: pytest tests/examples/

Closes #2496