[primer] Clarify location diff wording and robustify key iteration

- format_span now returns `line:col`-`endLine:endCol` spans, and
  message_diff says "Moved from {old} to {new}" — avoids ambiguity
  between span-range and movement.
- message_diff iterates set(old) | set(new) so keys present only in
  new are also detected.

Author: Pierre-Sassoulas

pylint/testutils/_primer/comparator.py

@@ -68,8 +68,8 @@ def format_span(msg: JSONMessage) -> str:
     end_line = msg.get("endLine")
     end_col = msg.get("endColumn")
     if end_line is not None and end_col is not None:
-        return f"from `{start}` to `{end_line}:{end_col}`"
-    return f"at `{start}`"
+        return f"`{start}`-`{end_line}:{end_col}`"
+    return f"`{start}`"
 
 
 def message_diff(old: JSONMessage, new: JSONMessage) -> str:
@@ -80,14 +80,14 @@ def message_diff(old: JSONMessage, new: JSONMessage) -> str:
     renders them with red/green highlighting.
     """
     changed_keys: set[str] = set()
-    for key in old:
-        if old[key] != new[key]:  # type: ignore[literal-required]
+    for key in set(old) | set(new):
+        if old.get(key) != new.get(key):
             changed_keys.add(key)
 
     parts: list[str] = []
     # Location: combine line/column/endLine/endColumn into one sentence.
     if changed_keys & _LOCATION_KEYS:
-        parts.append(f"Was raised {format_span(old)}, now {format_span(new)}.")
+        parts.append(f"Moved from {format_span(old)} to {format_span(new)}.")
 
     # Other fields (typically ``message`` or ``type``).
     for key in sorted(changed_keys - _LOCATION_KEYS):

tests/testutils/_primer/cases/line_moved/expected.txt

@@ -8,7 +8,7 @@ Changed messages:
 <details>
 
 1) [too-complex](https://github.com/pylint-dev/astroid/blob/123456789abcdef/astroid/node_classes.py#L103):
-Was raised from `100:0` to `100:15`, now from `103:0` to `103:15`.
+Moved from `100:0`-`100:15` to `103:0`-`103:15`.
 </details>
 
 *This comment was generated for commit v2.14.2*

tests/testutils/_primer/cases/message_changed_line/expected.txt

@@ -8,7 +8,7 @@ Changed messages:
 <details>
 
 1) [too-complex](https://github.com/pylint-dev/astroid/blob/123456789abcdef/astroid/node_classes.py#L103):
-Was raised from `100:0` to `100:15`, now from `103:0` to `103:15`.
+Moved from `100:0`-`100:15` to `103:0`-`103:15`.
 
 ```diff
 - 'my_function' is too complex. The McCabe rating is 18

tests/testutils/_primer/cases/mixed_changes/expected.txt

@@ -14,7 +14,7 @@ Changed messages:
                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 ```
 2) [too-complex](https://github.com/pylint-dev/astroid/blob/123456789abcdef/astroid/node_classes.py#L105):
-Was raised from `100:0` to `100:15`, now from `105:0` to `105:15`.
+Moved from `100:0`-`100:15` to `105:0`-`105:15`.
 
 confidence: `UNDEFINED` → `HIGH`
 

tests/testutils/_primer/cases/multi_package/expected.txt

@@ -8,7 +8,7 @@ Changed messages:
 <details>
 
 1) [too-complex](https://github.com/pylint-dev/astroid/blob/123456789abcdef/astroid/node_classes.py#L105):
-Was raised from `100:0` to `100:15`, now from `105:0` to `105:15`.
+Moved from `100:0`-`100:15` to `105:0`-`105:15`.
 
 ```diff
 - 'my_function' is too complex. The McCabe rating is 18