Fix Local Testing implementation of to_date and to_char with corrected type hints

[sfc-gh-stan]

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1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes #NNNN

2. Fill out the following pre-review checklist:

   • [ ] I am adding a new automated test(s) to verify correctness of my new code
   • [ ] I am adding new logging messages
   • [ ] I am adding a new telemetry message
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   • [ ] I am adding a new dependency

3. Please describe how your code solves the related issue.

   Local Testing implementation of the built-in SQL functions uses type hints to determine whether a value should be passed as a column or a single literal value. to_date and to_char had incorrect type hints before (essentially, format could also be a column of string instead of a single literal string), the fixes in #1187 caused regression in Local Testing implementation of mock_to_date and mock_to_char. This PR fixes that.