[codex] Refactor api access lifecycle helpers

[tkilias]

What changed

This PR applies the structural api_access.py refactor on top of PR 3.

It includes:

• the _api_access internal package split
• the slim exasol/saas/client/api_access.py facade
• the allowed-IP cleanup that accompanies the refactor
• the unit-test updates needed for the new patch targets and structure

Why it changed

The previous PRs establish the final behavior. This PR keeps that behavior while reorganizing the implementation into smaller internal modules so the lifecycle helpers are easier to reason about and maintain.

Impact

There should be no intended user-facing behavior change relative to PR 3. The impact is internal structure and maintainability.

Validation

• poetry run pytest -q test/unit/test_api_access.py test/unit/test_ensure_type.py
• poetry run nox -s format:check
• poetry run nox -s lint:typing

[sonarqubecloud]

Quality Gate failed

Failed conditions
2 New Code Smells (required ≤ 0)

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