fix: disable regex nosubs|optimize flags on MSVC

[dranger003]

Fixes a regression introduced by #17786 where models with complex tokenizer patterns (e.g., gpt-4o) fail to load on Windows/MSVC with error_stack.

MSVC's std::regex has severe stack limitations with nested lookaheads. The nosubs | optimize flags exacerbate this, causing failure during pattern compilation.

This PR uses #ifdef _MSC_VER to apply the flags only on platforms where they're beneficial (GCC/Clang on Linux/macOS) while preserving the original behaviour on Windows.

Closes #17830

[dranger003]

The problem: The original fix works on Linux/GCC but breaks Windows/MSVC because MSVC's std::regex implementation is fundamentally flawed for complex patterns.

Why MSVC Fails

The gpt-oss tokenizer regex is:

[^\r\n\p{L}\p{N}]?((?=[\p{L}])([^a-z]))*((?=[\p{L}])([^A-Z]))+(?:'[sS]|...|'[dD])?|...

This pattern contains:

1. Multiple lookaheads — (?=[\p{L}])
2. Nested capturing groups — ((?=...)([^a-z]))*
3. Complex Unicode properties — \p{L}, \p{N}

MSVC's std::regex implementation uses recursive backtracking that can easily exhaust the call stack. The nested method calls in MSVC's std::_Matcher are insane, and this is a known design issue that causes stack overflows for complicated expressions.

The error_stack error code specifically means "There was insufficient memory to determine whether the regular expression could match the specified character sequence."

Platform	Without PR #17786	With PR #17786
Linux/GCC	Crashes on repetitive input at runtime	✅ Works
Windows/MSVC	✅ Works for normal input	Crashes at model load

The nosubs and optimize flags change how MSVC's regex engine processes the pattern. Paradoxically, these "optimization" flags seem to trigger a different code path in MSVC that hits the stack limit earlier during pattern preparation/compilation rather than during matching.

Solutions

1. Add a preprocessor conditional to only apply the nosubs|optimize flags on non-MSVC platforms.

2. Replace std::regex with a more robust regex library (like RE2, PCRE2, etc.) - but this introduces potentially unwanted external dependencies.