Fix inplace_stop_source request_stop return value

[fallintoplace]

This fixes the boolean result returned by inplace_stop_source::request_stop().

The implementation currently returns true when a stop request was already made and false after this call successfully requests stop. That is reversed. The first call should return true, and later calls should return false.

The regression test checks both calls and confirms that the stop request is visible.

Tests:

• cmake --build build --target test.exec -j 4
• build/test/exec/test.exec '[stop_token]'
• build/test/exec/test.exec

[copy-pr-bot]

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