Implement Spark-compatible `xpath_double` / `xpath_number`

Start in crates/spark/src/function/xml/ and confirm whether the shared evaluator from the xpath issue already exists; this issue only adds evaluate_number(xml, path) -> Option<f64> on top of it, plus registration of xpath_number as an alias. Cross-read Spark's XPathDoubleEvaluator/UDFXPathUtil.evalNumber for the number() conversion rules, then write crates/sqllogictest/test_files/xml/xpath_double.slt covering the node-set, boolean, NaN/Infinity, -0 and NULL cases listed here. Done means both names pass the slt file and the function carries a #[user_doc] entry.

Part of #271 and apache/datafusion-comet#7300.

What the function does

xpath_double(xml, path) evaluates an XPath 1.0 expression, converts the result with the XPath number()
rules (XPathConstants.NUMBER, a Java Double), and returns it as DOUBLE. xpath_number is a registered
alias of the same expression: expression[XPathDouble]("xpath_number", true), and its pretty name is
xpath_number. Both arguments are strings (collation-aware from 4.0), and path must be foldable. The result
is always nullable. Available since Spark 2.0.0.

Parsing, DTD and entity handling, namespaces, NULL and empty-string handling, invalid-XML and invalid-path
errors, and lazy path compilation are shared with the family. They are specified in the xpath issue, and
this function should reuse that evaluator. This issue covers only the numeric conversion. xpath_float,
xpath_int, xpath_long and xpath_short take the same double and narrow it.

Examples

SELECT xpath_double('<a><b>1</b><b>2</b></a>', 'sum(a/b)');
-- 3.0
SELECT xpath_double('<a>this is not a number</a>', 'a');
-- NaN
SELECT xpath_double('<a>this 2 is not a number</a>', 'a');
-- NaN
SELECT xpath_double('<a><b>2000000000</b><c>40000000000</c></a>', 'a/b * a/c');
-- 8.0E19
SELECT xpath_double('<a>try a boolean</a>', 'a = 10');
-- 0.0
SELECT xpath_double('<a><b class="odd">1</b><b class="even">2</b><b class="odd">4</b><c>8</c></a>', 'a/b');
-- 1.0
SELECT xpath_double('<a><b class="odd">1</b><b class="even">2</b><b class="odd">4</b><c>8</c></a>', 'sum(a/*)');
-- 15.0
SELECT xpath_double('<a><b class="odd">1</b><b class="even">2</b><b class="odd">4</b><c>8</c></a>', 'sum(a/b[@class="odd"])');
-- 5.0
SELECT xpath_number('<a><b>1.0</b><b>2.1</b></a>', 'sum(a/b)');
-- 3.1

These come from the ExpressionDescription, the Hive udf_xpath_double golden files run by
HiveCompatibilitySuite, XPathExpressionSuite and XPathFunctionsSuite. In UDFXPathUtilSuite, a/c[2]
on <c>-77</c> gives -77.0, and an attribute k="foo" gives NaN.

Edge cases to test

• Conversion rules from XPath 1.0 number():
  • a node-set uses the string value of its first node in document order (a/b above gives 1.0, not the sum);
  • a boolean gives 1.0 or 0.0 (a = 10 above);
  • a string is parsed with XPath's grammar. Surrounding whitespace is allowed, as are an optional leading -
    and digits with an optional .. There is no exponent and no leading +. Anything else is NaN, for
    example '1e3', '+1', '1,000', 'Infinity' and '0x10'.
• Empty node-set. The usage string says "zero if no match is found", but XPath 1.0 converts an empty node-set
  through number(''), which is NaN. Verify xpath_double('<a/>', 'a/b') against Spark before asserting
  either value (the integer variants map NaN to 0 anyway).
• NaN, Infinity and -Infinity can be produced by arithmetic (0 div 0, 1 div 0, -1 div 0) and must
  come back as the IEEE values. -0 (-0 * 1) should keep its sign bit.
• Large and imprecise values: 8.0E19 above, and sum() over many decimal strings, which uses double
  arithmetic (1.0 + 2.1 gives 3.1 in the suite).
• NULL or '' for either argument gives NULL. A non-literal path gives DATATYPE_MISMATCH.NON_FOLDABLE_INPUT.
  Malformed XML and an invalid path raise runtime errors (see the xpath issue). ANSI mode has no effect.
• xpath_number must return the same values and show xpath_number as its display name.

Differences across Spark versions

None found beyond the shared ones in the xpath issue.

Implementation notes

• Spark: XPathDouble in sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/xml/xpath.scala.
  It calls XPathDoubleEvaluator (.../xml/XmlExpressionEvalUtils.scala), which calls
  UDFXPathUtil.evalNumber. The xpath_number alias is in FunctionRegistry.scala.
• DataFusion core has no equivalent.
• Build it on the shared evaluator from the xpath issue in crates/spark/src/function/xml/. A single
  evaluate_number(xml, path) -> Option<f64> helper can serve xpath_double, xpath_float, xpath_int,
  xpath_long and xpath_short. Register xpath_number as an alias.
• Tests: crates/sqllogictest/test_files/xml/xpath_double.slt, which also covers xpath_number. Document
  the function with #[user_doc].

This issue was generated by an LLM (Claude) from the Apache Spark source code and tests. Examples come from Spark's ExpressionDescription examples, test suites and .sql.out golden files, but please verify them against Spark before relying on them.