fix: marshall multi-byte strings correctly

[bdraco]

We needed to calculate the size based on the bytes string not the utf-8 string

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (f9e5d1d) 83.37% compared to head (362da93) 83.25%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #261 +/- ## ========================================== - Coverage 83.37% 83.25% -0.13% ========================================== Files 28 28 Lines 3290 3290 Branches 682 682 ========================================== - Hits 2743 2739 -4 - Misses 331 333 +2 - Partials 216 218 +2 ``` | [Files](https://app.codecov.io/gh/Bluetooth-Devices/dbus-fast/pull/261?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bluetooth-Devices) | Coverage Δ | | |---|---|---| | [src/dbus\_fast/\_private/marshaller.py](https://app.codecov.io/gh/Bluetooth-Devices/dbus-fast/pull/261?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bluetooth-Devices#diff-c3JjL2RidXNfZmFzdC9fcHJpdmF0ZS9tYXJzaGFsbGVyLnB5) | `93.37% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/Bluetooth-Devices/dbus-fast/pull/261/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bluetooth-Devices)

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