Implement XPath abbreviate of parentheses and fix some bugs

[tompng]

Implement abbreviate of parenthesized :group added in #317

Also fixes these bugs so that we can test parenthesized xpaths such as (1+2)*3

parser = REXML::Parsers::XPathParser.new
parser.abbreviate parser.parse("count(/a)") #=> NoMethodError
parser.abbreviate parser.parse("-1") #=> NoMethodError
parser.abbreviate parser.parse("1+2") #=> NoMethodError
parser.abbreviate parser.parse("/a[1+2]") #=> "/a[1 plus 2]" (XPath doesn't have `plus` keyword)

[Copilot]

Pull request overview

Note

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Extends the XPath abbreviation logic to cover additional XPath expression node types (arithmetic, grouping, unary negation) and adds tests to validate the new behavior.

Changes:

• Added parser tests for literals, unary/binary operators, grouping/parentheses, functions, and unknown nodes.
• Updated abbreviate to delegate more node types to predicate_to_path.
• Expanded predicate_to_path to format +, -, *, unary negation, and grouping nodes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
test/parser/test_xpath.rb	Adds test coverage for arithmetic/grouping/function formatting and unknown-node handling.
lib/rexml/parsers/xpathparser.rb	Updates abbreviation/serialization logic to support more parsed XPath node types.

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[tompng]

This is not good, but we need to do this to avoid infinite recursion of unknown operator.
predicate_to_path calls abbreviate if predicate_to_path doesn't know the operator.

Some possible workaround for this may break XPathParser#expand which is not tested and the spec is unknown.