fix: format behavior of union types

[amyXia1994]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y
  re [Enhancement][Track] Formatting tools with KCL Code style specifications #627

2. What is the scope of this PR (e.g. component or file name):

fix the to string impl of union type:

• kclvm/ast/src/ast.rs

test cases:
kclvm/tools/src/format/test_data/format_data/union_types.golden
kclvm/tools/src/format/test_data/format_data/union_types.input
kclvm/tools/src/format/tests.rs

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

fix: format behavior of union types

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Other

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• N
• Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• Unit test
• Integration test
• Benchmark (add benchmark stats below)
• Manual test (add detailed scripts or steps below)
• Other

[Peefy]

LGTM

[coveralls]

Pull Request Test Coverage Report for Build 6574118919

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.82%

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Totals
Change from base Build 6562245967:	0.0%
Covered Lines:	2300
Relevant Lines:	2619

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💛 - Coveralls