fix(tool): handle undefined input while streaming

[minwookshin]

Closes #490.

Initial input-streaming input can be undefined, which makes JSON.stringify return undefined and crashes CodeBlock at code.split. Return null from ToolInput until input exists so the surrounding tool stays visible. Preserve JSON rendering for defined values, including null, false, 0, an empty string, and an empty object.

Add eight regression cases covering missing input, partial-to-available input, clearing previous parameters, and defined falsy values. Update the example and prop documentation to show the initial streaming state. No public API changes.

Validation:

• Reproduced the original code.split crash in Chromium before applying the fix.
• Full test suite: 47 files, 966 tests passed in Chromium on Node.js 22.14.0.
• Production documentation build passed: pnpm build --filter ai-elements --filter docs.
• Changed TSX files pass the formatter invoked by Ultracite and repository-pinned oxlint.
• Desktop and mobile previews display the Pending header without parameters and render JSON when a populated tool is expanded.

Repository-wide checks have existing limitations: pnpm check flags nine untouched files; the Ultracite lint invocation fails to resolve import/no-unresolved. Standalone elements TypeScript checking reports the same 57 diagnostics with both HEAD and this patch, with none in changed files. The documentation preview also emits a React hydration warning on both Tool and the untouched Suggestion page.

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