Fix undefined array key crash in FunctionDocblockManipulator

[eyupcanakman]

Fixes #11793

For abstract and interface methods, the function-body slice in FunctionDocblockManipulator::__construct excludes the trailing ;. The return-type scanner can reach the last index of $chars while still inside the word-char branch, and accessing $chars[$i + 1] triggers an Undefined array key warning that Psalm's error handler turns into a RuntimeException.

Guard the access with isset(). The sibling / and * branches aren't reachable at end of scan.

---

Note

Low Risk
Low risk bugfix that adds a bounds check to prevent a runtime exception when manipulating return types on abstract/interface methods; behavior should be unchanged except for avoiding the crash.

Overview
Prevents FunctionDocblockManipulator from reading past the end of the return-type token stream by guarding $chars[$i + 1] with isset(), fixing a crash when processing methods whose signatures end with ; (e.g., interface/abstract methods).

Adds regression coverage in ReturnTypeManipulationTest for interface methods (including namespaced return types) where fixing MismatchingDocblockReturnType should remove the redundant @return docblock without errors.

^{Reviewed by Cursor Bugbot for commit bc877d8. Bugbot is set up for automated code reviews on this repo. Configure here.}