test: Add resource provider integration tests

[mashawes]

Fills in the missing integration test coverage for the resource provider, following the existing test patterns and harness.

• Domain lookups: find_domain_by_name (found and missing) and get_domain_enabled.
• Project lookups: get_project_by_name (found and missing) and get_project_parents (verifying a parent/child hierarchy).
• Negative cases: creating a domain or project with an over-length name is rejected. Resource delete is idempotent, it returns Ok for a missing id, so a delete-not-found error case isn't applicable; get-not-found is already covered.
• All tests use the existing harness (get_state, create_domain!, create_project!) and the public provider API via ExecutionContext; no production code is changed.

Closes #448

Note: this contribution was developed with AI assistance.

[gtema]

• from now on you need to satisfy the DCO (https://github.com/apps/dco) which is literally just a "Signed-Off-By: ..." trailer in the commit message. You can do this by using "-s" option to git commit: "git commit -s"
• Use the existing create_domain! / create_project! macros for consistency
• Add a second negative‑validation test that attempts to create a domain/project with an empty name and asserts is_err()
• Consider adding a test that creates a grand‑child (project → parent → grand‑parent) and checks that the full ancestor chain is returned, confirming that the provider correctly walks multiple levels.