feat(bigframes): Transpiler supports more string ops

[TrevorBergeron]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request adds support for compiling f-strings, nullity checks (such as 'is None' and 'is not None'), and various string operations (e.g., capitalize, islower, isupper) in Python UDFs. Feedback is provided regarding the Polars compilation of 'IsLowerOp' and 'IsUpperOp', where the current regex patterns incorrectly require the entire string to consist only of letters, failing to match Python's native behavior which allows non-cased characters.

[gemini-code-assist]

In Python, str.islower() and str.isupper() return True if there is at least one cased character and all cased characters are lowercase/uppercase respectively. Non-cased characters (such as spaces, punctuation, and digits) are allowed.

The current regex patterns ^[a-z]+$ and ^[A-Z]+$ require the entire string to consist only of letters of that case, which incorrectly returns False for valid Python strings like "hello world", "hello!", or "abc123".

To match Python's behavior, use regex lookaheads to ensure there is at least one cased character of the correct case, and no cased characters of the opposite case.

Suggested change

	@compile_op.register(string_ops.IsLowerOp)
	def _(self, op: ops.ScalarOp, input: pl.Expr) -> pl.Expr:
	assert isinstance(op, string_ops.IsLowerOp)
	return input.str.contains(r"^[a-z]+$")
	@compile_op.register(string_ops.IsUpperOp)
	def _(self, op: ops.ScalarOp, input: pl.Expr) -> pl.Expr:
	assert isinstance(op, string_ops.IsUpperOp)
	return input.str.contains(r"^[A-Z]+$")
	@compile_op.register(string_ops.IsLowerOp)
	def _(self, op: ops.ScalarOp, input: pl.Expr) -> pl.Expr:
	assert isinstance(op, string_ops.IsLowerOp)
	return input.str.contains(r"^(?=[^a-z]*[a-z])[^A-Z]+$")
	@compile_op.register(string_ops.IsUpperOp)
	def _(self, op: ops.ScalarOp, input: pl.Expr) -> pl.Expr:
	assert isinstance(op, string_ops.IsUpperOp)
	return input.str.contains(r"^(?=[^A-Z]*[A-Z])[^a-z]+$")