[spark] Case-insensitive field matching when casting nested struct on write.

[JunRuiLee]

Purpose

When writing BY NAME into a Paimon table whose nested struct field names differ from the source only in case, the insert fails with "field does not exist", because the lookup is case-sensitive and ignores spark.sql.caseSensitive.

Example:

CREATE TABLE t1 (id INT, info STRUCT<Age: INT, Name: STRING>);
CREATE TABLE t2 (id INT, info STRUCT<name: STRING, age: INT>);
INSERT INTO t1 VALUES (1, struct(30, 'Alice'));
INSERT INTO t2 BY NAME SELECT * FROM t1;  -- fails

Tests

• insert by name with case-insensitive nested struct field matching
• insert by name with case-insensitive matching inside array<struct<...>>
• insert by name with case-insensitive matching inside nested struct
• insert by name rejects ambiguous source nested struct fields
• insert by name rejects nested struct target fields colliding under resolver