fix: `datetime.datetime` instances in stream maps are now correctly mapped to `date-time` JSON schema strings

[edgarrmondragon]

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📚 Documentation preview 📚: https://meltano-sdk--2741.org.readthedocs.build/en/2741/

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #2741 will not alter performance

_{Comparing 2739-bug-customstreammap-always-convert-datetime-field-to-date (e1e7988) with main (5bb2d85)}

Summary

✅ 6 untouched benchmarks

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.67%. Comparing base (ce6941f) to head (e1e7988). Report is 4 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2741 +/- ## ======================================= Coverage 90.67% 90.67% ======================================= Files 62 62 Lines 5021 5021 Branches 641 641 ======================================= Hits 4553 4553 Misses 326 326 Partials 142 142 ```

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