Add ExprUsedAsStringNode virtual node emitted when expressions are used as strings

[phpstan-bot]

Summary

Implements a new ExprUsedAsStringNode virtual node that is emitted whenever an expression is used in a string context. This allows rule authors to hook into a single node type instead of having to handle many different AST node types (Echo_, Print_, Cast\String_, BinaryOp\Concat, InterpolatedString, AssignOp\Concat, InlineHTML) individually.

Changes

• Added src/Node/ExprUsedAsStringNode.php — new virtual node extending NodeAbstract with getExpression() (the expression being used as string) and getOriginalNode() (the AST node causing the string usage)
• src/Analyser/NodeScopeResolver.php — emit for each echo expression and for InlineHTML statements (using TypeExpr wrapping a ConstantStringType)
• src/Analyser/ExprHandler/PrintHandler.php — emit for the print expression
• src/Analyser/ExprHandler/CastStringHandler.php — emit for (string) cast inner expression
• src/Analyser/ExprHandler/InterpolatedStringHandler.php — emit for interpolated strings and heredocs
• src/Analyser/ExprHandler/AssignOpHandler.php — emit for .= concat assignment right-hand side

For concat chains used inside echo/print/cast/.= contexts, a single ExprUsedAsStringNode is emitted wrapping the entire concat expression — solving the issue reporter's complaint about getting multiple rule invocations per line for nested BinaryOp\Concat nodes.

Root cause

There was no unified virtual node for "expression used as string" contexts. Rule authors had to hook into Node::class and use instanceof checks for every relevant AST node type, with special handling needed for BinaryOp\Concat chains to avoid duplicate errors.

Test

Added tests/PHPStan/Rules/ExprUsedAsString/ with:

• ExprUsedAsStringTestRule.php — test rule that reports each ExprUsedAsStringNode emission
• ExprUsedAsStringRuleTest.php — verifies correct emission for all covered contexts: echo (single and multi-expression), print, (string) cast, interpolated string, concat assignment, echo with concat chain, heredoc, and inline HTML

Fixes phpstan/phpstan#13008