test: Add WASM text metric update coverage (R3.46)

[khangnghiem]

This PR adds unit test coverage for the WASM-side update_text_metrics API, fulfilling requirement R3.46.

What

• Added tool_update_text_metrics_updates_bounds
• Added tool_update_text_metrics_ignores_non_text_nodes
• Added tool_update_text_metrics_with_max_width
• Added tool_update_text_metrics_with_parent_managed
• Updated docs/REQUIREMENTS.md test matrix to reflect coverage.

Why

To improve test coverage for the WASM API bridge and satisfy the requirement R3.46 which was previously untested directly.

Impact

Improves confidence in the text measuring logic within the WASM module without changing actual behavior.

Measurement

Run cargo test --workspace to execute the newly added 4 unit tests.

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PR created automatically by Jules for task 8318446655495389111 started by @khangnghiem

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