feat: add xpath(xpathStr, xmlStr) CEL function

[Copilot]

Adds XPath 1.0 evaluation support to the CEL expression engine, consistent with the existing jmespath, jsonpath, and jq functions.

Changes

• coll/json.go – XPath(xpathStr, xmlStr string) ([]string, error) core implementation via github.com/antchfx/xmlquery; always returns a []string of text contents for all matching nodes (empty slice for no matches)
• funcs/coll_gen.go – CEL binding collXPathGen with signature xpath(string, string) → dyn
• funcs/cel_exports.go – registers collXPathGen in CelEnvOption
• funcs/coll.go – exposes xpath for Go template use alongside jmespath/jsonpath

Usage

xpath('//name/text()', '<root><name>Alice</name></root>').first()  // "Alice"
xpath('//item/@id', '<root><item id="42"/></root>').first()         // "42"
xpath('//items/item', xml)                                          // ["a", "b", "c"] for multiple matches
xpath('//missing', '<root/>')                                       // []

[moshloop]

@copilot xpath should always return a list, use xpath('//item/@id', '<root><item id="42"/></root>').first()

[Copilot]

@copilot xpath should always return a list, use xpath('//item/@id', '<root><item id="42"/></root>').first()

Done in 58327ce. xpath now always returns []string, so callers can use .first() for a single value:

xpath('//item/@id', '<root><item id="42"/></root>').first()  // "42"
xpath('//item/@id', '<root><item id="1"/><item id="2"/></root>')  // ["1", "2"]