Fix array spread traversable compliance + JIT test (#4686)

[PurHur]

Summary

• Fix array_spread_traversable.phpt: remove ...null block that expected catchable TypeError; Zend/php-src and ArraySpread use uncatchable fatal (#4812).
• Add array_spread_null_fatal.phpt for null spread fatal semantics.
• Add array_spread_traversable_jit.phpt guarding JIT execute parity for IteratorAggregate, Generator, and Iterator spread (lowering already in HashTableHelper::spreadInto + JitIteratorToArray).

Closes #4686

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
sed -n "/^--FILE--$/,/^--EXPECT--$/p" test/compliance/cases/language/array_spread_traversable.phpt | tail -n +2 | head -n -1 > /tmp/spread_trav.php
php /tmp/spread_trav.php
php bin/vm.php /tmp/spread_trav.php
php bin/jit.php /tmp/spread_trav.php
./vendor/bin/phpunit --filter array_spread
'

Output (VM/JIT match Zend for traversable operands):

array (
  'a' => 1,
  'b' => 2,
)
array (
  0 => 10,
  1 => 20,
)
array (
  0 => 1,
  1 => 2,
  2 => 3,
)

PHPUnit:

OK (16 tests, 20 assertions)

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