[upstream] Update PlatformIO to 6.1.18

This is part of a MR series updating PlatformIO to v6.1.18. This request updates the upstream branch.

Discussion at https://lists.debian.org/debian-python/2025/08/msg00034.html

PlatformIO uses git submodules, thus upstream changes are merged manually.

# Command used to generate this MR
git remote add upstream https://github.com/platformio/platformio-core.git
git fetch upstream tag v6.1.18
git merge --no-ff --no-edit v6.1.18

See also:

• !3
• !4