Fix: Prevent infinite loop from circular extends in config

[SpacePlushy]

Summary

Fixes #5096 - Resolves infinite loop when a config section extends itself (directly or indirectly).

Problem

Previously, the walk_options method in platformio/project/config.py did not check for circular dependencies when processing extends directives. This caused infinite loops in cases like:

[base]
extends = base

or more complex chains:

[common]
extends = base

[base]
extends = common

Solution

Added cycle detection by checking if a section is already in extends_done or extends_queue before adding it to the processing queue. This prevents sections from being processed multiple times and breaks circular dependency chains.

Changes

• Modified walk_options method in platformio/project/config.py (lines 180-188)
• Changed from extends_queue.extend() to iterating and checking each section individually
• Added check: if ext_section not in extends_done and ext_section not in extends_queue

Testing

Tested with both simple and complex circular reference cases:

1. Simple circular: [base] extends = base
2. Complex circular: [common] extends = base + [base] extends = common

Both cases now complete successfully without hanging, while still preserving valid config inheritance behavior.

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