Fix goto_definition range end and add textDocument/definition integration tests

- Fix LspPosition.from_ast_pos_end usage for range end in goto_definition
- Add integration test suite with four test scenarios:
  class members (fields, params, methods), trait-typed receiver (one
  definition), union-typed receiver (multiple definitions), and
  cross-file navigation

Author: orien

.release-notes/5123.md

@@ -0,0 +1,3 @@
+## Fix goto_definition range end
+
+The `textDocument/definition` response now returns a correct `range.end` position. Previously, it had an off-by-one error in the column value.