fix: _set_status should not override existing span status (#3713)

[RiyaChaturvedi37]

Description

Fixes #3713

Problem

_set_status was unconditionally calling span.set_status(Status(status))
without checking if a status was already set. This caused two bugs:

• A previously set ERROR status could be downgraded to OK
• A previously set status description was wiped out (overridden with None)

Fix

Added a priority check before calling span.set_status:

• Never downgrade status (ERROR → OK or ERROR → UNSET are blocked)
• Preserve existing description if same status code and no new description provided

Tests

Added 4 new tests in test_semconv_status.py:

• test_does_not_downgrade_error_to_ok
• test_does_not_wipe_description_with_none
• test_upgrades_unset_to_error
• test_unset_to_ok