Truncate type display for long unions in some situations

[AlexWaygood]

Summary

One of the diagnostics in the ecosystem report for astral-sh/ruff#20368 is:

[error] invalid-argument-type - :1121:30 - Argument is incorrect: Expected `str`, found `Unknown | str | ((sd: @Todo | SpectralDistribution | MultiSpectralDistributions, cmfs: MultiSpectralDistributions | None = None, illuminant: SpectralDistribution | None = None, k: @Todo | None = None, method: str = str, **kwargs: Any) -> @Todo) | ((XYZ: @Todo, method: str = str, **kwargs: Any) -> SpectralDistribution) | ((sd: SpectralDistribution, lef: SpectralDistribution | None = None) -> int | float) | ((XYZ: @Todo) -> @Todo) | ((Y: @Todo) -> @Todo) | ((LV: @Todo, method: str = str, **kwargs: Any) -> @Todo) | partial[Unknown] | ((wavelength: @Todo, cmfs: MultiSpectralDistributions | None = None) -> @Todo) | ((xyY: @Todo) -> @Todo) | ((xy: @Todo) -> @Todo) | ((Lab: @Todo) -> @Todo) | ((Luv: @Todo, illuminant: @Todo = Any) -> @Todo) | ((uv: @Todo) -> @Todo) | ((UVW: @Todo) -> @Todo) | ((Lab_99: @Todo, illuminant: @Todo = Any, k_E: int | float = int, k_CH: int | float = int, method: str = str) -> @Todo) | ((Lab_hdr: @Todo, illuminant: @Todo = Any, Y_s: @Todo = float, Y_abs: @Todo = int, method: str = str) -> @Todo) | ((ICaCb: @Todo) -> @Todo) | ((ICtCp: @Todo, illuminant: @Todo = Any, chromatic_adaptation_transform: @Todo | str | None = str, method: str = str, L_p: int | float = int) -> @Todo) | ((IgPgTg: @Todo) -> @Todo) | ((IPT: @Todo) -> @Todo) | ((XYZ_D65: @Todo, constants: Structure = Structure) -> @Todo) | ((Jzazbz: @Todo, constants: Structure = Structure) -> @Todo) | ((IPT_hdr: @Todo, Y_s: @Todo = float, Y_abs: @Todo = int, method: str = str) -> @Todo) | ((Ljg: @Todo, optimisation_kwargs: dict[Unknown, Unknown] | None = None) -> @Todo) | ((ProLab: @Todo, illuminant: @Todo = Any) -> @Todo) | ((Iab: @Todo) -> @Todo) | ((Yrg: @Todo) -> @Todo) | ((uvV: @Todo) -> @Todo) | ((uvL: @Todo, illuminant: @Todo = Any) -> @Todo) | ((RGB: @Todo) -> @Todo) | ((HSV: @Todo) -> @Todo) | ((HSL: @Todo) -> @Todo) | ((HCL: @Todo, gamma: int | float = int, Y_0: int | float = int) -> @Todo) | ((HYS: @Todo) -> @Todo) | ((CMY: @Todo) -> @Todo) | ((CMYK: @Todo) -> @Todo) | ((Lrgb: @Todo) -> @Todo) | ((YCbCr: @Todo, K: @Todo = Any, in_bits: int = int, in_legal: bool = bool, in_int: bool = bool, out_bits: int = int, out_legal: bool = bool, out_int: bool = bool, clamp_int: bool = bool, **kwargs: Any) -> @Todo) | ((YcCbcCrc: @Todo, in_bits: int = int, in_legal: bool = bool, in_int: bool = bool, is_12_bits_system: bool = bool, **kwargs: Any) -> @Todo) | ((YCoCg: @Todo) -> @Todo) | ((value: @Todo, function: @Todo | str = str, **kwargs: Any) -> @Todo) | ((HEX: @Todo) -> @Todo) | ((keyword: str) -> @Todo) | ((xyY: @Todo, hue_decimals: int = int, value_decimals: int = int, chroma_decimals: int = int) -> str | @Todo) | ((munsell_colour: @Todo) -> @Todo) | (Overload[(sd_test: SpectralDistribution, additional_data: bool = bool, method: str = EllipsisType) -> ColourRendering_Specification_CRI, (sd_test: SpectralDistribution, *, additional_data: bool, method: str = EllipsisType) -> int | float, (sd_test: SpectralDistribution, additional_data: bool, method: str = EllipsisType) -> int | float]) | (Overload[(sd_test: SpectralDistribution, *, additional_data: bool, method: str = EllipsisType) -> int | float, (sd_test: SpectralDistribution, additional_data: bool = bool, method: str = EllipsisType) -> ColourRendering_Specification_CQS, (sd_test: SpectralDistribution, additional_data: bool, method: str = EllipsisType) -> int | float]) | ((CCT_D_uv: @Todo) -> @Todo) | ((mired: @Todo) -> @Todo) | ((specification: CAM_Specification_CIECAM02) -> @Todo) | ((JMh: @Todo) -> CAM_Specification_CIECAM02) | ((specification: CAM_Specification_CAM16) -> @Todo) | ((JMh: @Todo) -> CAM_Specification_CAM16) | ((specification: CAM_Specification_CIECAM16) -> @Todo) | ((JMh: @Todo) -> CAM_Specification_CIECAM16) | ((specification: CAM_Specification_Hellwig2022) -> @Todo) | ((JMh: @Todo) -> CAM_Specification_Hellwig2022) | ((specification: CAM_Specification_sCAM) -> @Todo) | ((JMh: @Todo) -> CAM_Specification_sCAM) | ((JMh: @Todo) -> @Todo) | ((Jpapbp: @Todo) -> @Todo)`

That's a ridiculously long display for a type, and it's needlessly long. The user has all the information they need after they're presented with the first union element that's not assignable to str. We should instead display something like:

[error] invalid-argument-type - :1121:30 - Argument is incorrect: Expected `str`, found `Unknown | str | ((sd: @Todo | SpectralDistribution | MultiSpectralDistributions, cmfs: MultiSpectralDistributions | None = None, illuminant: SpectralDistribution | None = None, k: @Todo | None = None, method: str = str, **kwargs: Any) -> @Todo) | (...30 union elements omitted)`

(I didn't actually count how many elements there are in that very long union, but you get the picture.)

Version

No response