Preserve sort keys during reflected SortedKeyList multiplication

[Eric3-jp]

SortedKeyList overrides __mul__, but inherits SortedList.__rmul__. As a result, multiplying on the left drops the custom sort key:

from operator import neg
from sortedcontainers import SortedKeyList

values = SortedKeyList([1, 2, 3], key=neg)
list(values * 2)  # [3, 3, 2, 2, 1, 1]
list(2 * values)  # currently [1, 1, 2, 2, 3, 3]

It can also raise TypeError for values such as dictionaries that require a key function to be ordered. Bind SortedKeyList.__rmul__ to its own __mul__, following the existing __radd__ pattern, so both multiplication directions preserve the key.

Add regression coverage for empty and nonempty inputs, negative/zero/positive counts, and dictionary values ordered by a custom key. The tests also verify that the original list is unchanged and repeated values retain their object identities.

Validation on Python 3.12 / Ubuntu 24.04:

• All nine new regression cases fail before the fix.
• Full pytest -q --tb=short: 375 passed, including the regression cases and doctests.
• Ruff formatting passes for both changed files.
• doc8, rstcheck, Sphinx HTML build, package build, and twine check pass.
• Ruff 0.16.8 reports five existing B905 diagnostics and formatting changes in two untouched files (docs/conf.py and src/sortedcontainers/sorteddict.py). The same results were confirmed in a clean checkout of the base commit; this patch adds no new diagnostics.

AI disclosure: This issue was identified during a code review, and the change was prepared and tested with OpenAI Codex.