pl: restore sum and product arity templates after the connector fix

[michaldziwisz]

Summary

Follow-up to #739 and the Polish verification report.

Now that the engine reads the connector from the language definitions instead of hard-coding English "of", restore the Polish arity templates for sum and product:

• Define FunctionApplicationWord: ["z"].
• Use function=suma | przez | od,do and function=iloczyn | przez | od,do.
• Update the four existing expectations that recorded the previous workaround.

For example, sum($i,$x) becomes "suma przez i z x" instead of "suma z i przecinek, x", and sum($i,$n,$x) becomes "suma od i do n z x". The corresponding product forms use "iloczyn". The one-argument forms remain unchanged.

The Polish wording follows the existing large-operator rules. This changes only Rules/Languages/pl/definitions.yaml and tests/Languages/pl/definitions.rs. No engine or braille changes; independent of #764.

Verification

Rechecked against current main at 374ea588cd77edc1745b8a632da1d23ec6e9c71f (crate 0.7.6-rc.4):

• Unmodified upstream: cargo test --test languages Languages::pl -j2: 625 passed, 0 failed.
• With this change, the same suite: 625 passed, 0 failed.
• With --features no-unsafe: 625 passed, 0 failed.
• Negative control: reverted only the Polish definitions, retaining the updated expectations. sequence_and_series_intents failed on the intended difference: expected "suma przez i z x", actual "suma z i przecinek, x". Restoring the definitions makes it pass again.
• git diff --check passes. The diff contains only the two files listed above.

[NSoiffer]

Thanks. I never would have guessed that "z" => "of".