fix: preserve default-input-method during key reads (upstream #52, #58)

Two reports from breadncup (issue #52 in 2023, issue #58 in 2024) said that running an org-drill session silently nulled out their `default-input-method`. The reproduction is exact: every rating prompt clears the user's persistent setting.

The cause is `(set-input-method nil)` in `org-drill--read-key-sequence`. When `current-input-method` is nil, calling `set-input-method` with nil clears `default-input-method` as a documented side effect. The unwind-protect on the way back has the symmetric problem, since it passes the captured nil. The fix is to use the primitives that are scoped to current state. `deactivate-input-method` and `activate-input-method` don't touch `default-input-method`, and I wrap each call in a guard so the function is a no-op when no input method is active.

The same pattern lives in `org-drill-response-get-buffer-create`, which propagates the caller's input method into the response buffer. When the caller has no input method active, the captured value is nil and `(set-input-method nil)` runs in the new buffer, clearing `default-input-method` again. I applied the same guard there.

I added `tests/test-org-drill-read-key-sequence.el` with 6 ERT tests across Normal, Boundary (the bug case), and Error categories. The four regression tests fail at HEAD before the fix and pass after.

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