fix: Raise on missing struct fields when casting with strict=True

[MatteoPossamai]

Closes #28587.

Problem

Struct casts map fields by name and unconditionally fill unmatched target
fields with nulls. Under strict=True this never raised:

s = pl.Series([{"a": 1, "b": 2}])
s.cast(pl.Struct({"x": pl.Int64, "y": pl.Int64}), strict=True)
# [{'x': None, 'y': None}]  <- no error, all data gone

Changes

• StructChunked::cast_impl: when a target field is not found in the
  source struct, raise a SchemaError if the cast is strict. The error
  message names the missing field and lists the available source fields.
  Non-strict casts keep the existing null-filling behavior
• test_nested_struct_cast_22744 now passes strict=False: it relies on
  null-filling missing fields, which remains supported for non-strict casts
  (per the expected behavior in cast(Struct{...}, strict=True) to renamed fields silently nulls every value #28587).
• Added test_struct_cast_strict_missing_field_28587 covering: strict cast
  raises for both zero-overlap and partial-overlap field names, and
  non-strict cast still null-fills.

Local test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.46%. Comparing base (ae588a9) to head (57db45b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #28631      +/-   ##
==========================================
- Coverage   81.33%   76.46%   -4.88%     
==========================================
  Files        1870     1870              
  Lines      263034   263039       +5     
  Branches     3217     3217              
==========================================
- Hits       213950   201144   -12806     
- Misses      48253    61064   +12811     
  Partials      831      831              

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