Apply feedback to v2 branch

[iMicknl]

Apply feedback form @tetienne in #1872.

Feedback on models.py skipped for now, as this will require a larger refactor (like https://github.com/iMicknl/python-overkiz-api/pulls).

[Copilot]

Pull request overview

This PR applies review feedback to the v2 branch by tightening model validation, improving auth error mapping for Nexity, enforcing enum fallback conventions, and aligning internal client request helpers/tests with updated method names.

Changes:

• Add validation to StateDefinition to require name or qualified_name, plus a unit test for this behavior.
• Rename internal HTTP helpers on OverkizClient (__get/__post/__delete → _get/_post/_delete) and update tests accordingly.
• Improve Nexity auth error handling by mapping specific Cognito ClientError codes to NexityBadCredentialsException while propagating other errors; add test coverage.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/test_models.py	Adds coverage for StateDefinition rejecting invalid payloads.
tests/test_client.py	Updates backoff-related tests to patch renamed internal client methods.
tests/test_client_queue_integration.py	Updates queue integration tests to patch _post instead of name-mangled __post.
tests/test_auth.py	Adds tests validating Nexity Cognito error mapping/propagation and updates imports for patching.
pyoverkiz/models.py	Enforces required identifier field for StateDefinition.
pyoverkiz/enums/base.py	Adds subclass-time enforcement that UnknownEnumMixin users define UNKNOWN.
pyoverkiz/client.py	Switches to _get/_post/_delete, improves cert path handling via Path, and updates docstring style in execute_action_group.
pyoverkiz/auth/strategies.py	Uses get_running_loop() and refines Nexity Cognito error mapping to avoid masking non-credential failures.
pyoverkiz/action_queue.py	Aligns docstring with Google-style sections used by the docs tooling.

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