fix(flutter): unblock analyzer with wildcard separator parameters

[lukisch]

Current-master Flutter run38798432885 stops at flutter analyze with exactly two unnecessary_underscores findings in playlist separatorBuilder callbacks, preventing the full suite from running. Replace the unused second parameter __ with the supported Dart wildcard _. Only these two callback parameter lists change; divider widgets and all behavior remain unchanged. Existing analyzer rules, tests and workflow remain enabled. Validation: exact-base failure observed; git diff --check passed. Flutter is unavailable locally; exact-head CI must run the library test, unchanged analyzer and full suite. No dependency, product version, release or device-acceptance change. Author model: gpt-6.1-sol (Codex). Independent model review pending. Draft; merges disabled.

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