Add parse/stringify round-trip test for TypeCast

[Copilot]

lp.TypeCast lacked a test verifying that stringification and re-parsing produce an identical expression.

Changes

• New test test_type_cast_parse_stringify_roundtrip: Creates a TypeCast expression, stringifies it via loopy's StringifyMapper, parses the result, and asserts equality.

expr = lp.TypeCast(np.float64, parse("x[i]"))
stringified = StringifyMapper()(expr)
# → "cast(np:dtype('float64'), x[i])"
parsed = parse(stringified)
assert expr == parsed

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[Copilot]

Pull request overview

Adds a regression test to ensure lp.TypeCast expressions can be stringified via Loopy’s StringifyMapper, parsed back, and remain structurally identical—guarding against future stringify/parser drift in Loopy’s symbolic layer.

Changes:

• Add test_type_cast_parse_stringify_roundtrip to validate TypeCast stringify→parse round-trip equality.

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