Added helper functions to check if a number is close to an integer and if it is a power of a base.

PR Title
Add helper functions for number checks and validations in ticker.py

PR Summary

This PR introduces two internal helper functions to lib/matplotlib/ticker.py to streamline numeric validations and improve code readability for future ticker enhancements.

Changes:

• Added _is_close_to_int(x): A utility using math.isclose to determine if a value is effectively an integer within a relative tolerance of $1e-9$.
• Added _is_decade(x, *, base=10, rtol=None): A utility to determine if a given value is an integer power of a specified base (e.g., $1, 10, 100$ for base 10). This handles finite checks and logarithmic comparisons with configurable relative tolerance.

Why is this change necessary?

These common checks are frequently needed when calculating tick positions and formatting labels, especially for logarithmic scales. Centralizing them as helper functions reduces code duplication and ensures consistent precision across the ticker module.

AI Disclosure

I used an AI assistant to help format this PR description based on the code changes I authored.

PR Checklist

• New and changed code is [tested](https://matplotlib.org/devdocs/devel/testing.html)
• Documentation complies with [general](https://matplotlib.org/devdocs/devel/document.html#write-rest-pages) and [docstring](https://matplotlib.org/devdocs/devel/document.html#write-docstrings) guidelines
• "closes #0000" is in the body (N/A - Internal utility improvement)