Add edge case tests for personal corpus feature

[JSv4]

Context

From PR #834 code review - the test suite covers main flows well but lacks specific edge case tests.

Suggested Tests

1. Concurrent Creation Race Condition

Verify that simultaneous calls to get_or_create_personal_corpus() from multiple threads don't create duplicates or raise unhandled errors.

def test_concurrent_personal_corpus_creation(self):
    from concurrent.futures import ThreadPoolExecutor
    
    user = User.objects.create_user(...)
    Corpus.objects.filter(creator=user, is_personal=True).delete()
    
    def create_corpus():
        return Corpus.get_or_create_personal_corpus(user)
    
    with ThreadPoolExecutor(max_workers=5) as executor:
        results = list(executor.map(lambda _: create_corpus(), range(5)))
    
    # All should return the same corpus
    self.assertEqual(len(set(c.pk for c in results)), 1)
    # Only one should exist
    self.assertEqual(Corpus.objects.filter(creator=user, is_personal=True).count(), 1)

2. Delete and Recreate Flow

Verify that deleting personal corpus and then uploading creates a new one correctly.

def test_personal_corpus_recreate_after_deletion(self):
    user = User.objects.create_user(...)
    original_corpus = Corpus.objects.get(creator=user, is_personal=True)
    original_corpus.delete()
    
    # Upload should create new personal corpus
    new_corpus = Corpus.get_or_create_personal_corpus(user)
    self.assertNotEqual(new_corpus.pk, original_corpus.pk)
    self.assertTrue(new_corpus.is_personal)

3. Embedding Task Queue Failure

Test behavior when ensure_embeddings_for_corpus.delay() fails to queue (e.g., Redis unavailable).

Location

opencontractserver/tests/test_personal_corpus.py

Priority

Low - existing tests cover happy paths, these are hardening tests.