fix(tool): trigger fallback on empty task output - #29048

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Closes #24447 as well.

Force-updated from current dev with hybrid TaskTool empty-output handling. Latest non-empty text still returns, legitimate tool-call/tool-only child runs return a diagnostic, and truly silent empty outputs still fail so fallback can catch them. Local verification: bun test test/tool/task.test.ts; bun typecheck from packages/opencode.

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