gh-137165: Unify datetime.strftime behavior for unsupported non-zero-padded specifiers

[F0RRZZ]

This PR unifies the behavior of datetime.strftime when encountering unsupported or invalid non-zero-padded specifiers across different platforms.

Previously, the behavior for invalid specifiers was inconsistent:
Windows: Raised a ValueError
macOS/FreeBSD: Could return partial strings or stripped characters (e.g., returning # instead of %-#).
Linux: Echoes the specifier back as-is (e.g., %-#).

The behavior is now unified to match Linux.

• Issue: Enhancement Proposal: Standardized Support for Non-Zero-Padded Day and Month Formatting in datetime.strftime #137165
• Related PR: gh-137165: Standardized non-zero-padded date formatting for datetime.strftime #139088

• Issue: Enhancement Proposal: Standardized Support for Non-Zero-Padded Day and Month Formatting in datetime.strftime #137165