[BugFix] Fix csv writer using incorrect number of columns

[letian-jiang]

Why I'm doing:

For cases like

select 1 as a, 1 as b 
into outfile "file:///tmp/test3.csv" 
format as csv;

The chunk has only one column. But two slots are required for output.

What I'm doing:

Use the size of output_exprs as the number of columns.

Fixes #issue

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 3.2
  • 3.1
  • 3.0
  • 2.5

[github-actions]

[FE Incremental Coverage Report]

✅ pass : 0 / 0 (0%)

[github-actions]

[BE Incremental Coverage Report]

❌ fail : 0 / 1 (00.00%)

file detail

	path	covered_line	new_line	coverage	not_covered_line_detail
🔵	src/exec/plain_text_builder.cpp	0	1	00.00%	[49]

[github-actions]

@Mergifyio backport branch-3.2

[github-actions]

@Mergifyio backport branch-3.1

[github-actions]

@Mergifyio backport branch-3.0

[mergify]

backport branch-3.2

✅ Backports have been created

• #38099 [BugFix] Fix csv writer using incorrect number of columns (backport #38045) has been created for branch branch-3.2

[github-actions]

@Mergifyio backport branch-2.5

[mergify]

backport branch-3.1

✅ Backports have been created

• #38100 [BugFix] Fix csv writer using incorrect number of columns (backport #38045) has been created for branch branch-3.1

[mergify]

backport branch-3.0

✅ Backports have been created

• #38101 [BugFix] Fix csv writer using incorrect number of columns (backport #38045) has been created for branch branch-3.0

[mergify]

backport branch-2.5

✅ Backports have been created

• #38102 [BugFix] Fix csv writer using incorrect number of columns (backport #38045) has been created for branch branch-2.5