Warning when extending from own env

[Unaimend]

PlatformIO Core, version 6.1.17

[base]
extends = base

Please don't ask how I found out, but this code results in pio being stuck without any warning

[SpacePlushy]

I've identified and fixed this issue! The problem was in the walk_options method in platformio/project/config.py - it didn't check for circular dependencies when processing extends directives.

The fix adds cycle detection to prevent sections from being added to the processing queue if they're already in extends_done or extends_queue.

Pull request: #5303

I've tested it with both simple circular references ([base] extends = base) and complex chains, and it now completes successfully without hanging.