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.

🤖 Generated with Claude Code

[ivankravets]

The PR contains a lot of changed files. Please push the only related file.

[mahdirajaee]

The cycle detection fix in config.py is concise: before appending to extends_queue, it checks if ext_section not in extends_done and ext_section not in extends_queue. This prevents infinite loops from circular extends chains (e.g., env:a extends env:b, env:b extends env:a). The approach is correct — extends_done tracks already-processed sections and extends_queue tracks pending ones, so checking both guarantees no section is visited twice.

One edge case to consider: this silently skips the circular reference rather than raising an error or warning. A user with an unintentional cycle in their platformio.ini will get no feedback that their config isn't being resolved as expected — the circular section will simply be ignored. Adding a click.secho warning or a log message when a cycle is detected would help with debuggability. Deeply nested but non-circular configs should be unaffected since extends_done only blocks re-visits.

Similar to PR #5302, this diff includes changes to piobuild.py (the -U flag handling) and test/cli.py (the hardcoded PASSED fix) that appear to be from separate concerns. Keeping each PR scoped to a single fix makes review and potential reverts cleaner.