Validate FixedString sizes before swapping

[fallintoplace]

What changed

• Reject swaps between different FixedString sizes before changing either column.
• Keep same-size swaps working as before.
• Add coverage for both valid and invalid swaps.

Tests

• cmake --build build --target clickhouse-cpp-ut --parallel 8
• ./build/ut/clickhouse-cpp-ut --gtest_filter='ColumnsCase.*'

[slabko]

Thank you @fallintoplace for noticing this inconsistency!