fix: correct false negative for BoardGameGeek (#2803)

[victoireantoine1707-design]

Issue #2803

Sherlock fails to detect existing BoardGameGeek profiles
The errorMsg was inverted: the API returns {"isValid":true} when a username
is AVAILABLE, and {"isValid":false} when it is TAKEN (i.e. user exists)

Solution proposed
Changed errorMsg from "isValid":true to "isValid":false to correctly
detect existing profiles

Verification done
Manually verified via API:

• https://api.geekdo.com/api/accounts/validate/username?username=zzznoneuser → {"isValid":true} (not found)
• https://api.geekdo.com/api/accounts/validate/username?username=blue → {"isValid":false,...} (found)

[github-actions]

Automatic validation of changes

Target	F+ Check	F- Check
BoardGameGeek	❌   Fail	❌   Fail

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Failures were detected on at least one updated target. Commits containing accuracy failures will often not be merged (unless a rationale is provided, such as false negatives due to regional differences).