chore: fallback for spark.sql.legacy.castComplexTypesToString.enabled = true

[comphead]

Which issue does this PR close?

Closes #4492

Rationale for this change

Spark's spark.sql.legacy.castComplexTypesToString.enabled flag changes how
array, map, and struct are formatted when cast to string:

Flag	Wrapping	NULL elements
false (default, Comet implements this)	{...} for maps/structs, [...] for arrays	rendered as the literal "null"
true (legacy)	[...] for maps/structs	omitted from output

In Spark 4.0 the flag is marked internal and defaults to false, but it is
still honored when set. Comet only implements the default formatting, so
running with the legacy flag on produces results that diverge from Spark.

What changes are included in this PR?

• CometCast.isSupported: when spark.sql.legacy.castComplexTypesToString.enabled=true,
  return Unsupported for any ArrayType/StructType/MapType → StringType
  cast so the plan falls back to Spark.
• Refactor several near-duplicate type-walking helpers into one generic
  SupportLevel.containsType(dt, classOf[T1], classOf[T2], ...) (replaces
  containsFloatingPoint, containsMapType, two local containsBinary).
• Extract the strict-floating-point gate into
  SupportLevel.strictFloatingPointReason(dt, what) so the five repeated
  STRICT_FP + containsFloatingPoint + boilerplate message blocks collapse to
  one call.
• SQL file tests:
  • cast_complex_types_to_string_legacy.sql — asserts struct/array/map and
    nested complex types fall back when the legacy flag is on.
  • cast_complex_types_to_string.sql — exhaustive coverage of struct → string
    and array → string across every supported field/element type
    (named/anonymous, ints at min/max, decimals at 38-precision, dates,
    timestamps, binary with non-printable bytes, NaN/±0/±Infinity, NULL at
    every depth, empty containers, deep nesting). Map → string queries assert
    expect_fallback(Cast from MapType) since map → string is not implemented
    natively.

[andygrove]

LGTM pending CI. Thanks @comphead.

[comphead]

cast from maptype to string needs to be improved