fix(native-tools): accept empty attempt_completion.result

[0xMink]

Refs #11404

Summary

• Change attempt_completion nativeArgs gating from truthiness (if (args.result)) to existence check (if (args.result !== undefined)) in both the streaming partial parse path and the finalize parse path in NativeToolCallParser.ts
• An empty string "" is falsy in JavaScript, so result: "" was incorrectly treated as missing, producing the "missing nativeArgs" error
• Add 4 regression tests: empty-string result, non-empty result, missing result, and streaming empty-string result

Test plan

• Existing: 12 parser tests continue to pass
• New: 4 tests for attempt_completion edge cases (empty string, non-empty, missing, streaming empty)
• Total: 16/16 passing

cd src && npx vitest run core/assistant-message/__tests__/NativeToolCallParser.spec.ts

[roomote]

Rooviewer   See task

No issues found. The fix correctly replaces a truthiness check with an explicit !== undefined check, handling the result: "" edge case that was silently dropping attempt_completion tool calls.

• No issues identified

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