[WFS] Fix filtering by QGIS expression in order to support "@geometry" (Fix #60094)

[agiudiceandrea]

Description

Adds support to @geometry (#50134) besides $geometry in WFS filtering by QGIS expression.

The isGeometryColumn() method has been updated to also recognize the @geometry variable in order for the expressionFunctionToOgcFilter() method to correctly traslate expressions containing the @geometry variable.
(The expressionFromOgcFilter() has not been updated to use @geometry instead $geometry)

Added tests for expressions using @geometry.
Also updated the WFS section of the QgsVectorLayer documentation (following up #60095) substituting $geometry with @geometry (ref. #60095 (comment) @uclaros).

Fixes #60094.

I'm unsure about the correctness of the error message at:

QGIS/src/core/qgsogcutils.cpp

Line 2385 in 42e7ad1

mErrorMessage = QObject::tr( "<BBOX> is currently supported only in form: bbox(@geometry, geomFromWKT('…'))" );

Should it be amended somehow?

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[rouault]

I'm not totally sure about that but couldn't there be situations where a complicated function would be used that would reference the "geometry" variable but also others, and thus would not just be "@geometry" ? If so, the following could be more appropriate:

Suggested change

	return fd->name() == QLatin1String( "$geometry" ) || fn->referencedVariables().contains( QLatin1String( "geometry" ) );
	return fd->name() == QLatin1String( "$geometry" ) || ( fn->referencedVariables().size() == 1 && fn->referencedVariables().contains( QLatin1String( "geometry" ) ) );

[agiudiceandrea]

You are right! Anyway, thinking about it more carefully, it looks like even fn->referencedVariables().size() == 1 && fn->referencedVariables().contains( QLatin1String( "geometry" ) ) wouldn't reject an expression like intersects(centroid(@geometry), geomFromWKT(‘POINT (5 6)’)) while it should be.
It seems to me it should be used instead: fd->name() == QLatin1String( "var" ) && fn->referencedVariables().contains( QLatin1String( "geometry" ) )
so we ensure that the first argument is actually a "var" QgsExpressionNode::ntFunction which can only reference 1 variable, and not a generic function that references a "geometry" variable.

[rouault]

t seems to me it should be used instead: fd->name() == QLatin1String( "var" ) && fn->referencedVariables().contains( QLatin1String( "geometry" ) )

yes that looks even better