fix: StringIndexOutOfBoundsException in presentation compiler's hasColon method (#23498)

This PR fixes a `StringIndexOutOfBoundsException` that could occur in
the presentation compiler when checking for colons in override
completions.

## Problem
The `dotty.tools.pc.completions.OverrideCompletions.hasColon` method was
accessing characters at specific indices without first verifying that
the indices were within the text bounds. This could cause crashes when
the span end positions were at or beyond the text length.

## Solution
Added bounds checking before accessing characters:
- For `TypeDef` cases: verify `td.rhs.span.end < text.length` before
`text.charAt(td.rhs.span.end)`
- For `Template` parent cases: filter to ensure `text.length > idx`
before `text.charAt(idx)`

## Testing
The existing test suite passes, and a new test case `no-new-line` was
added to cover the edge case.

[resolves
scalameta/metals#7575](scalameta/metals#7575)

Author: m-gris

presentation-compiler/src/main/dotty/tools/pc/completions/OverrideCompletions.scala

@@ -511,10 +511,10 @@ object OverrideCompletions:
       Context
   ): Option[Int] =
     defn match
-      case td: TypeDef if text.charAt(td.rhs.span.end) == ':' =>
+      case td: TypeDef if (td.rhs.span.end < text.length) && text.charAt(td.rhs.span.end) == ':' =>
         Some(td.rhs.span.end)
       case TypeDef(_, temp : Template) =>
-        temp.parentsOrDerived.lastOption.map(_.span.end).filter(text.charAt(_) == ':')
+        temp.parentsOrDerived.lastOption.map(_.span.end).filter(idx => text.length > idx && text.charAt(idx) == ':')
       case _ => None
 
   private def fallbackFromParent(parent: Tree, name: String)(using Context) =

presentation-compiler/test/dotty/tools/pc/tests/edit/AutoImplementAbstractMembersSuite.scala

@@ -75,39 +75,59 @@ class AutoImplementAbstractMembersSuite extends BaseCodeActionSuite:
          |""".stripMargin
     )
 
-  @Test def `empty-lines-between-members` =
+  @Test def `no-new-line` =
     checkEdit(
       """|package a
-         |
-         |object A {
-         |  trait Base {
-         |    def foo(x: Int): Int
-         |    def bar(x: String): String
-         |  }
-         |  class <<Concrete>> extends Base {
-         |
-         |    def bar(x: String): String = ???
-         |
-         |  }
-         |}
-         |""".stripMargin,
+          |
+          |trait X:
+          |  def foo: Unit
+          |
+          |class <<Y>> extends X""".stripMargin,
       """|package a
          |
-         |object A {
-         |  trait Base {
-         |    def foo(x: Int): Int
-         |    def bar(x: String): String
-         |  }
-         |  class Concrete extends Base {
-         |
+         |trait X:
+         |  def foo: Unit
          |
-         |    override def foo(x: Int): Int = ???
+         |class Y extends X {
          |
-         |    def bar(x: String): String = ???
+         |  override def foo: Unit = ???
          |
-         |  }
-         |}
-         |""".stripMargin
+         |}""".stripMargin,
+    )
+
+  @Test def `empty-lines-between-members` =
+    checkEdit(
+      """|package a
+        |
+        |object A {
+        |  trait Base {
+        |    def foo(x: Int): Int
+        |    def bar(x: String): String
+        |  }
+        |  class <<Concrete>> extends Base {
+        |
+        |    def bar(x: String): String = ???
+        |
+        |  }
+        |}
+        |""".stripMargin,
+      """|package a
+        |
+        |object A {
+        |  trait Base {
+        |    def foo(x: Int): Int
+        |    def bar(x: String): String
+        |  }
+        |  class Concrete extends Base {
+        |
+        |
+        |    override def foo(x: Int): Int = ???
+        |
+        |    def bar(x: String): String = ???
+        |
+        |  }
+        |}
+        |""".stripMargin
     )
 
   @Test def `objectdef` =