test: Add regex filter e2e tests (#49984)

## Summary

Add Python client E2E coverage for regex filter expressions (`=~` and
`!~`).

Coverage includes:
- Basic regex semantics, negation, Unicode, empty values, invalid
patterns, and unsupported RE2 constructs
- VarChar, JSON, ARRAY element, NULL, and unsupported field type
behavior
- Query, search, hybrid search, boolean combinations, limit/offset,
delete, and concurrency correctness
- Scalar index paths including NGRAM, INVERTED, STL_SORT, BITMAP, Trie,
sealed/growing segments, and reload
- Template expression parameters for regex filters, including invalid
regex/type mismatch/injection safety

Related issue: #49899

## Test Plan

- [x] `./tests/python_client/.venv/bin/python -m ruff check
tests/python_client/milvus_client/test_milvus_client_regex_filter.py`
- [x] `./tests/python_client/.venv/bin/python -m py_compile
tests/python_client/milvus_client/test_milvus_client_regex_filter.py`
- [x] `./.venv/bin/pytest
milvus_client/test_milvus_client_regex_filter.py --uri
http://localhost:19530 --tag L1 -n 4 --dist loadscope -x -v -s
--tb=short` (`68 passed, 544 warnings in 131.52s`)

---------

Signed-off-by: lyyyuna <yiyang.li@zilliz.com>