fix(slide): tolerate malformed generated slide data

[yipwingtim]

Summary

Improve slide renderer robustness when generated slide data contains malformed or missing fields.

This prevents client-side render failures caused by common bad generated data, such as missing text
values, invalid table cell spans, or unsafe numeric values.

Changes

• Normalize text-like values before rendering.
• Guard table cell text and span values.
• Avoid invalid shape transform values from malformed dimensions.
• Keep rendering behavior unchanged for valid slide data.

Verification

• Cherry-picked onto latest upstream/main.
• TypeScript/lint/build verified in downstream branch before preparing the upstream PR.

[yipwingtim]

This is a defensive renderer fix for malformed AI-generated slide data. It keeps valid slide rendering unchanged and prevents client crashes when generated content misses optional text/span/dimension fields.