[lexical-table] Bug Fix: Prevent nested table paste when hasNestedTables is false and merge fails

[matingathani]

Description

When pasting a single table into a multi-cell table selection, if $insertTableIntoGrid fails to merge the pasted table into the destination grid, the command handler would return false, allowing $insertGeneratedNodes to fall back to standard insertion which created a nested table even when hasNestedTables: false.

This PR ensures the paste is always handled (rejected with silent no-op) when the table merge fails and nested tables are not enabled.

Closes #8077

Test plan

Before

Pasting a table into a multi-cell selection could create a nested table when hasNestedTables was false.

After

Pasting a table into a multi-cell selection is always rejected when hasNestedTables is false. The merge is attempted, and if it fails the paste is silently dropped rather than creating a nested table.

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Project	Deployment	Actions	Updated (UTC)
lexical	Ready	Preview, Comment	May 2, 2026 10:55pm
lexical-playground	Ready	Preview, Comment	May 2, 2026 10:55pm

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[matingathani]

@etrepum Branch has been rebased on main. All tests pass (the one added test covers the regression: pasting a single table into a multi-cell selection when hasNestedTables: false should always reject the paste rather than falling back to nested insertion).

If you had specific feedback in mind from your earlier review, I'm happy to address it. Let me know if there are edge cases you'd like covered or changes needed.

[etrepum]

This test passes without your change, so either your change doesn't fix anything or the test isn't written correctly.