fix: Improve code quality in ivy_guc.c

[OreoYang]

Summary

This PR improves code quality in src/backend/utils/misc/ivy_guc.c by fixing multiple issues identified during code review.

Changes

Performance Improvements

• Optimize nls_case_conversion() function: Cache strlen() result in a local variable to avoid O(N²) complexity, improving performance to O(N)
  • Previously: strlen() called on every loop iteration
  • Now: Called once at the start of each conversion cycle

Code Quality Fixes

• Replace magic numbers with character constants: Improve readability by using 'a', 'z', 'A', 'Z' instead of ASCII codes (97, 122, 65, 90)

Bug Fixes

• Fix typo in comment (line 59): "blow" → "below"
• Fix typo in gettext string (line 423): "charater" → "character"
• Fix typo in error message (line 480): Remove double period
• Remove trailing whitespace in comment (line 187)

PostgreSQL Standards Compliance

• Add missing errcode() to ereport() calls (lines 582, 584-585, 604-605):
  • Use ERRCODE_STRING_DATA_RIGHT_TRUNCATION for parameter length validation
  • Use ERRCODE_INVALID_PARAMETER_VALUE for NLS errors
  • Ensures proper error handling following PostgreSQL error reporting standards

Test Plan

✅ All regression tests passed:

• PostgreSQL Tests: 228 tests passed
• Oracle Tests: 235 tests passed (including Oracle-specific tests)
• Oracle Extension Tests: contrib/ivorysql_ora tests passed

Files Changed

• src/backend/utils/misc/ivy_guc.c (+25, -17)

Impact

• No functional changes to GUC parameter behavior
• Improves code maintainability and readability
• Optimizes performance of character case conversion
• Ensures PostgreSQL error reporting standards compliance