Fix IOS token version test under CI environment override

[Sripadvallabh]

Description

Isolate TestAbstractTokenDiscoveryConnection.test_ios_learn_tokens_from_show_version from the global LEARN_OS_VERSION environment override.

The test expects the full IOS version 15.0(20100325:222114). If the runner sets LEARN_OS_VERSION=False, version learning is disabled and the discovered version remains 15.

Use patch.dict(os.environ) to temporarily remove the override during the connection and assertions, then restore the original environment automatically. This follows the existing IOS and IOS XE version-learning test pattern and preserves the full-version assertion.

Validation

• Reviewed the diff and confirmed the patch matches the internal repository.
• git diff --check passed.
• Local tests have not been run for this follow-up; CI validation is pending.
• Targeted check: run test_utils.py::TestAbstractTokenDiscoveryConnection::test_ios_learn_tokens_from_show_version with LEARN_OS_VERSION=False and with the variable unset.

Includes a test-fix changelog.

[Sripadvallabh]

CI review before merging this follow-up test fix:

The failed PR job installs public unicon-26.8 and fails 12 existing connection-dialog tests that require Unicon 26.9 core methods and arguments. The push workflow reports the same failures. test_connection_dialog_override.py is unchanged by this PR; the PR updates environment isolation in the IOS version test in test_utils.py and adds its changelog.

The identical internal test fix is merged, and all its reported CI checks passed. The external failure is the existing prepublication dependency mismatch, not a failure in the code changed by this follow-up PR. Other external matrix cancellations are fail-fast cancellations and are not being described as passing tests.

The release lead explicitly authorized merging this external PR with the documented dependency exception. Matching Unicon 26.9 dependencies are required to validate the full external suite.