interactiveui: make getchar() testable

[anushamukka-dev]

Summary:
interactiveui.getchar() had a # TODO: figure out tests on its non-tty
early return, and no test coverage at all - there is no existing test file
for interactiveui anywhere in eden/scm.

This extracts the raw terminal read into a module-level _readraw(fd) helper
and gives getchar() two optional injectable seams:

• stdin - defaults to sys.stdin
• readraw - defaults to _readraw

Real terminal behavior is unchanged: production callers pass neither argument,
_readraw contains the identical tcgetattr / setraw / os.read /
tcsetattr sequence, and the os.isatty(fd) guard and ctrl-c / ctrl-d
handling are byte-for-byte the same.

Initializing ch = None before the try also resolves the three
pyre-fixme[61] suppressions. Those flagged a real latent bug: if
tcgetattr raised termios.error, the except branch read ch before it
was ever assigned and would have raised NameError instead of falling back
to "".

Adds tests/test-interactiveui.py covering the non-tty early return (backed
by a real pipe fd), a single normal keypress, ctrl-c / ctrl-d, and the escape
sequence branch for arrow keys - including a run of several arrow keys plus a
normal key delivered in one read, checked through _splitkeypresses. The tty
cases use a real pty fd so the os.isatty guard is exercised for real rather
than mocked, with only the blocking read faked. The suite follows the existing
silenttestrunner convention used by neighboring tests such as
test-extutil.py and needs no BUCK change - unittestify.py globs
test-*.py.

Differential Revision: D115572436

[meta-codesync]

@anushamukka-dev has exported this pull request. If you are a Meta employee, you can view the originating Diff in D115572436.