Extract ProjectHandler from CalciteRelNodeVisitor

[ykmr1224]

Description

• Extract ProjectHandler from CalciteRelNodeVisitor
• This is pure refactoring, and no logic change.
• I will raise some more PRs to reduce the size of CalciteRelNodeVisitor

Related Issues

n/a

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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[LantaoJin]

I will raise some more PRs to reduce the size of CalciteRelNodeVisitor

LGTM for Project. But I don't think the the size of CalciteRelNodeVisitor is a problem. Some methods can be shared in CalciteRelNodeVisitor. No need to create one handler per visit(). And small method sometimes is bad smell code. For example:

    if (isSingleAllFieldsProject(node)) {
      return handleAllFieldsProject(node, context);
    }

is confused.

    if ( node.getProjectList().size() == 1
        && node.getProjectList().getFirst() instanceof AllFields ) {
      return handleAllFieldsProject(node, context);
    }

looks better.

[ykmr1224]

I will raise some more PRs to reduce the size of CalciteRelNodeVisitor

LGTM for Project. But I don't think the the size of CalciteRelNodeVisitor is a problem. Some methods can be shared in CalciteRelNodeVisitor. No need to create one handler per visit(). And small method sometimes is bad smell code. For example:

I am not planning to extract all visitor to separate handlers. Just a few of them are huge and better extracted.
My main motivation is to control the context size for AI agent. It tend to fill up the context when touching CalciteRelNodeVisitor. Smaller context would improve AI agent's quality, even when it won't fill the context.

And small method sometimes is bad smell code.

isSingleAllFieldsProject is from before this change. And I think this kind of extraction is normally good one for readability.