Avoid composite for fixed IDNA test data

[Mirochill]

Summary

• replace the fixed mock_idna_text composite strategy with just(...)
• drop the now-unused composite-only imports from the Hypothesis tests

Closes #187.

Validation

• Local tests were not run because this workspace is intentionally coordination-only.
• Static validation only: git diff --check.
• Relying on the repository's GitHub Actions checks for remote validation.