fix: Fixing the issue of deactivating user while seat allocation

[rgopalrao-sonata-png]

Summary:
Jira ticket: https://2u-internal.atlassian.net/browse/ENT-12003
Guard against inadvertent enterprise link deactivation on re-assignment

Before calling create_pending_enterprise_users, create_pending_enterprise_learner_for_assignment_task now checks whether the learner is already actively linked to the target enterprise. If so, it records a successful linked action and returns early — skipping the LMS call that would otherwise trigger PendingEnterpriseCustomerUserSerializer's "inactivate other customers" side-effect.

Changes:

lms_client.py: new get_enterprise_learner_by_email() helper — resolves an email to an LMS user ID, then fetches the enterprise-learner record
tasks.py: pre-flight active-link check with early return; moves get_normalized_metadata_for_assignment import to content_metadata_api where it belongs
test_tasks.py: covers the skip path (active link), the proceed path (inactive/missing active field), and patches the new check in setUp so existing tests are unaffected

Test results:

[codecov]

Codecov Report

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.46%. Comparing base (22ade54) to head (6a6e5ad).

Files with missing lines	Patch %	Lines
enterprise_access/apps/api_client/lms_client.py	93.75%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #215      +/-   ##
==========================================
+ Coverage   86.44%   86.46%   +0.01%     
==========================================
  Files         154      154              
  Lines       12848    12869      +21     
  Branches     1227     1232       +5     
==========================================
+ Hits        11107    11127      +20     
  Misses       1426     1426              
- Partials      315      316       +1     

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