fix: read /source and editor files as UTF-8 - #2432

Follow-up to #2429: the same locale-encoding bug in the two remaining read paths (/source and the \e editor file).

Description

/source <file> (execute_from_file in client_commands.py) and the \e <file> editor read (open_external_editor in io_commands.py) opened the file with the locale's default encoding. Under a non-UTF-8 locale (Windows cp1252/cp932, LANG=C) a UTF-8 SQL file containing non-ASCII text is mis-decoded or rejected. The config and history files already pass encoding='utf-8'; these two open() calls were missing it.

Fix: pass encoding='utf-8' in both places, plus a changelog entry.

Tests: two new tests run the code in a subprocess with -X warn_default_encoding -W error::EncodingWarning, which fails on main (EncodingWarning: 'encoding' argument not specified) and passes with the fix. The existing open mocks in test_client_commands.py now accept the extra keyword.

Disclosure: this change was prepared with the help of an AI coding assistant (Claude); the submitter is responsible for it.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the mycli/AUTHORS.txt file (or it's already there).
• ruff check and ruff format --check pass on the touched files; mypy not run (only an encoding keyword was added).