fix: get_json_object returns first value for duplicate keys to match Spark

[u70b3]

Which issue does this PR close?

Closes #4947.

Rationale for this change

For a JSON object with duplicate keys, Spark's GetJsonObjectEvaluator stops at the first matching field, but Comet's native implementation resolved the key to its last occurrence (matching serde_json's overwrite semantics rather than Spark's):

SELECT get_json_object('{"a":1,"a":2}', '$.a');
-- Spark: 1
-- Comet (before this PR): 2

What changes are included in this PR?

SegmentVisitor::visit_map in native/spark-expr/src/string_funcs/get_json_object.rs now locks in the first key match and consumes later occurrences via IgnoredAny instead of re-parsing them with PathSeed:

• The lock is keyed on the key match, not on a successful subpath resolution: if the first matching value does not contain the rest of the path, the result is null and the second duplicate key is never consulted, matching Spark.
• All entries are still visited, so the existing malformed/trailing-garbage rejection behavior is unchanged.

How are these changes tested?

• Flipped the existing test_duplicate_key_last_wins unit test to test_duplicate_key_first_wins.
• Added test_duplicate_key_first_wins_nested ({"a":{"b":1},"a":{"b":2}} / $.a.b -> 1).
• Added test_duplicate_key_first_match_locks_value ({"a":{"x":1},"a":{"b":2}} / $.a.b -> null, matching Spark's lock-on-first-match semantics).
• Full datafusion-comet-spark-expr suite passes (497 tests), plus clippy and fmt checks.
• Checked spark/src/test/resources/sql-tests/ and CometJsonJvmSuite for duplicate-key cases: the only repeated keys there are across different objects inside arrays, which are unaffected by this change.