feat: Validate schema members against polars DataType usage

[Copilot]

• Add validation check in SchemaMeta.__new__ to detect schema members that are instances of pl.DataType
• Add validation check in SchemaMeta.__new__ to detect schema members that are types (subclasses) of pl.DataType
• Add test cases for both error scenarios (instance and type)
• Run tests to verify the changes work correctly (259 tests passed)
• Run code review to ensure quality and fix feedback
• Run security scan with CodeQL (no alerts found)
• Verify edge cases and comprehensive testing complete
• Refactor checks into single if statement per reviewer feedback

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fail creating schemas with members of type pl.DataType or type[pl.DataType]</issue_title>
<issue_description>### Problem

Similarly to previously reported schema member errors (#105, #126), a related issue occurs when a user defines a schema member as an instance of pl.DataType or its type, e.g.:

class MySchema(dy.Schema):
    name = pl.String()  # user error: pl.String is a polars DataType, not a Dataframely column

When this happens, the member is silently ignored/dropped in validation, so the resulting dataframe lacks this column with no warning or error, making debugging difficult. This is likely an unintentional user mistake, similar to cases already addressed in [#105] and [#126].

Suggested improvement

Consider raising an error when a schema contains members that are instances/types of pl.DataType, to help users catch such mistakes early.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Fail creating schemas with members of type pl.DataType or type[pl.DataType] #245

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b813ac9) to head (7473528).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #246   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           53        53           
  Lines         3066      3070    +4     
=========================================
+ Hits          3066      3070    +4     

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