[test-helpers] Add EuiTableObject

[steliosmavro]

Summary

Adds EuiTableObject for the low level EuiTable, the most used EUI selectors in Kibana tests today (euiTableRow, euiTableCell, about 108 references across Scout and FTR). EuiBasicTableObject could not be used there because its guard is .euiBasicTable.

• EuiTableObject: guard .euiTable, members rows, headerCells, column(index), cell(row, column). Positional access since a plain table has no field names.
• EuiBasicTableObject now extends EuiTableObject, mirroring EuiBasicTable rendering an EuiTable. It keeps its own guard and cells(field), its rows getter is now inherited with the same logic.
• Selectors under selectors.table, spec against the EuiTable Playground story, README.

QA

• Table and basic table specs pass against Storybook (11 tests).
• tsc clean.

[infra-vault-gh-plugin-prod]

💚 Build Succeeded

• Buildkite Build
• Commit: 664ae7d
• Documentation website
• Storybook
• 🚫 Visual regression tests skipped: No VRT-relevant paths changed

History

• 💚 Build #5890 succeeded c31f17c

[infra-vault-gh-plugin-prod]

💚 Build Succeeded

• Buildkite Build
• Commit: 664ae7d