fix: preserve nested mapping values in assertion diffs

[rudra337-lgtm]

What was wrong

When an assertion compared mappings whose differing values were lists, pytest used saferepr for the entire one-item mapping. Its nested repr truncation could hide the actual differing list element, even for a short eight-item list.

What changed

Render differing mapping entries with pprint.pformat, retaining the existing safe-repr fallback when formatting user objects raises. Added a regression test for the reported nested-list case.

Verification

• Baseline reproduction: the eight-item lists were shown as [1, 1, 1, 1, 1, 1, ...] on both sides.
• Focused assertion tests: 2 passed, 207 deselected.
• git diff --check passed.

Fixes #11980

[ege-arhan]

When differing mapping values wrap across multiple lines (e.g. nested lists or dictionaries with 10+ items exceeding pprint width), highlighter(left_repr) + " != " + highlighter(right_repr) glues != directly onto the last line of left_repr and embeds unhandled \n characters:

d1 = {"key": [f"item_{i}" for i in range(15)]}
d2 = {"key": [f"item_{i}" for i in range(14)] + ["diff"]}
# asserting d1 == d2 formats as:
# {key: [item_0,
#          ...,
#          item_14]} != {key: [item_0,
#          ...,
#          diff]}

Here the closing bracket of the left mapping shares a line with != and the first line of the right mapping, and the raw newlines bypass terminal reporter line prefixing/indentation. If multiline pretty-printing is introduced here, differing items should either split lines cleanly across separate blocks or preserve single-line rendering.