ENH: add hint for linear fill method error

[jiangmuran]

Summary

• add a hint when method="linear" is passed to fill methods so the error message points toward integer interpolation

Automated tools

• Used AI assistance and prompted it to follow AGENTS.md.

Checklist

• closes ENH: Better error message when interpolating dtype integer with method="linear" #41565
• Tests added and passed
• All code checks passed
• Added type annotations to new arguments/methods/functions
• Added an entry in the latest whatsnew file
• I have reviewed and followed all the contribution guidelines
• If I used AI to develop this pull request, I prompted it to follow AGENTS.md