🐛 Bug: String case conversion functions produce inconsistent results across environments

Description

The toTitle(), toLower(), and toUpper() functions use icu::Locale::getDefault(), which reads system locale settings. This causes different outputs in different environments (dev vs staging vs production).

Environment

• ICU Version: [e.g., 70.1]
• OS: Linux
• Container: Docker/Kubernetes
• Locale Settings: Varies (LANG=C vs LANG=en_US.UTF-8)

Steps to Reproduce

1. Run the following code in an environment with LANG=en_US.UTF-8:

std::string result = toTitle("a.b,c");
// Output: "A.b,C"

2. Run the same code in a container with LANG=en_US_POSIX:

std::string result = toTitle("a.b,c");
// Output: "A.B,C"

Expected Behavior

The functions should produce consistent, deterministic output regardless of system locale settings, matching Spark SQL behavior:

toTitle("a.b,c") → "A.b,C" (always)
toLower("TITLE") → "title" (always)
toUpper("title") → "TITLE" (always)

Actual Behavior

Output varies based on environment variables (LC_ALL, LC_CTYPE, LANG):

✅ LANG=en_US.UTF-8: toTitle("a.b,c") → "A.b,C"
❌ LANG=en_US_POSIX: toTitle("a.b,c") → "A.B,C"

Root Cause

Using icu::Locale::getDefault() makes the behavior environment-dependent.

Proposed Solution

Replace icu::Locale::getDefault() with icu::Locale::getRoot() in:

toTitle()
toLower()
toUpper()