Preserve regex syntax in case-insensitive DeepSearch

[Eric3-jp]

DeepSearch(['ABC', '123'], r'\D+', use_regexp=True) currently reports root[1]: its default case-insensitive handling lowercases the pattern, turning \D into \d. The same problem affects other uppercase regex escapes and scoped case-sensitive groups, including searches of dictionary paths.

Preserve the regex pattern and the text it searches, and use re.IGNORECASE for case-insensitive matching. Literal string searches retain their current handling. Tests cover uppercase escapes, named Unicode characters, scoped inline flags, dictionary paths, and explicit case sensitivity.

Validation on Python 3.12.3:

• Before the fix, 8 new regression cases failed and 8 controls passed; after the fix, all 78 search tests passed.
• Broader tests, including slow tests: 1,230 passed and 8 skipped. The ordinary serialization tests passed separately: 90 passed. Security-specific tests were excluded from these runs.
• search.py reached 100% coverage in the broader suite.
• pyright deepdiff/search.py: no errors or warnings.
• CI's blocking Flake8 selection and git diff --check: passed.

Prepared with OpenAI Codex assistance for implementation, regression tests, and review.