[arminazimi]

Summary

CopyFrom was flattening multidimensional arrays sent as PostgreSQL text-format
literals (e.g. "{{1,2},{3,4}}"). The issue is reproducible: inserting the same
literal with Exec preserves array_ndims = 2, while inserting it with CopyFrom
gives array_ndims = 1.

Root cause: when CopyFrom needs to binary-encode a value it cannot encode directly,
it falls back into tryScanStringCopyValueThenEncode, which rescans the text literal
into *any. pgtype.ArrayCodec.DecodeValue then flattens the array literal into a
single one-dimensional []any, so the COPY protocol binary payload carries
ndim = 1 even for multidimensional inputs.

This PR makes that fallback path preserve the original array dimensions by scanning
the text literal into pgtype.Array[any] (an ArraySetter) when the target column
is an array type, then re-encoding that value in binary. CopyFrom now produces the
same stored shape that Exec does.

Fixes #2385

Changes

• pgx/values.go: tryScanStringCopyValueThenEncode now uses pgtype.Map.TypeForOID
  and typechecks the returned *pgtype.Type.Codec against *pgtype.ArrayCodec before
  falling back to the any scan path. Dimension-preserving scan only applies to array
  target columns; the any fallback still handles non-array types.
• pgx/values_internal_test.go: new DB-free internal test
  TestEncodeCopyValuePreservesArrayDimensions verifying encodeCopyValue produces a
  2-D binary array header from a "{{1,2},{3,4}}" string and a 1-D header from
  "{a,b,c}", with NULL-element handling and non-array-type passthrough.
• pgx/copy_from_test.go: new integration test
  TestConnCopyFromPreservesMultidimensionalArrayString that inserts the same 2-D
  text[][] literal via both Exec and CopyFrom and asserts pgtype.Array[any].Dims,
  array_ndims(), array_dims(), array_length(b, 1), and NULL handling all match.

Test plan

• New integration test TestConnCopyFromPreservesMultidimensionalArrayString on
  PostgreSQL 16: inserts the same 2-D text[][] literal via both Exec and
  CopyFrom, then asserts dimensions, array_ndims(), array_dims(),
  array_length(b, 1), NULL elements, and NULL rows all match.
• New DB-free internal test TestEncodeCopyValuePreservesArrayDimensions:
  asserts encodeCopyValue produces a 2-D binary array header from a
  "{{1,2},{3,4}}" string and a 1-D header from "{a,b,c}", with NULL-element
  handling and non-array-type passthrough.
• Existing TestConnCopyFromAutomaticStringConversion re-run and still passes:
  CopyFrom still auto-converts "42" to int8 42 rows.
• Existing TestConnCopyFromAutomaticStringConversionArray re-run and still
  passes: [][]string / []string CopyFrom rows still round-trip through numeric
  arrays correctly.
• go test ./... -race re-run against the changed package; no new races.
• gofmt -d clean.