perf: cache grid property in HTMLTableSerializer

[ddoron9]

Problem

Nested grid[i][j] access in HTMLTableSerializer.serialize caused performance issues on large tables.

Change

Replaced double indexing with row-level iteration (for row in grid: for cell in row:). Fixes #372

Impact

Removes repeated Pydantic lookups and improves serialization speed without changing output.

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Related Documentation
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Documentation updates
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[vagenas]

Thanks for the contribution @ddoron9!
I just fixed some minor autoflake finding — make sure to install the pre-commit hooks so they automatically run for your next contribution 😉
Cheers!