Fix undefined behavior in StringView trimming and validate positive number parsing

[jdymitarai]

Summary of Changes

1. Fix undefined behavior in StringView trimming:

   • In standard C (C99 7.4 / C11 7.4), the argument to <ctype.h> functions like isspace() must be representable as unsigned char or equal to EOF.
   • When char is signed (default on x86/x64 MSVC and GCC/Clang), non-ASCII characters (>= 0x80) produce negative integer values when passed to isspace(), causing undefined behavior or debug assertions (e.g. _ASSERTE((unsigned)(c + 1) <= 256) on MSVC).
   • Cast characters to (unsigned char) before invoking isspace().

2. Reject empty views in ParsePositiveNumberWithBase:

   • Previously, parsing "0x" stripped the "0x" prefix leaving an empty remainder view (size == 0). The loop in ParsePositiveNumberWithBase never iterated and returned result = 0. As a result, the malformed literal "0x" was incorrectly treated as valid integer 0 instead of returning -1.
   • Returning -1 if view.size == 0 ensures incomplete hex literals without digits properly fail.

3. Prevent signed integer overflow in ParsePositiveNumberWithBase:

   • Added bounds check (result > (INT_MAX - value) / base) before result = (result * base) + value to guard against signed integer overflow on oversized strings.

4. Unit Tests:

   • Added test cases in test/string_view_test.cc for non-ASCII whitespace trimming, "0", "0x0", "0x", malformed hex suffixes, and overflow inputs.

Verification

All 17 unit tests in string_view_test and all CMake test suites pass cleanly on MSVC.