validationFailureLogMessage() was an abstract hook whose only job was to return a
cosmetically different warning string per provider ("Binance credential validation
failed", "Indexa Capital token validation failed", ...). It carried no functional
value and forced five one-line overrides.
Log a single structured message in the base with the provider enum value as
context, which is more useful for filtering than the free-text variants, and
remove the hook and its five overrides. Also restore the array-shape PHPDoc on
IndexaCapital's buildPendingAccounts() that the shared mixed signature dropped.
Only the warning-log text changes (now includes a "provider" key); the HTTP
responses are untouched. OpenBanking feature tests pass (304/304).
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