refactor: extract toggle-state helpers shared by F9 and F12

The F12 commit (554b32d) flagged this as a follow-up: ~120 lines of capture-state and display-saved logic were duplicated between modules/ai-vterm.el and modules/eshell-vterm-config.el. The only differences were the default direction (right for F9, below for F12) and the customization name for the fallback size. Extract the shared logic into modules/cj-window-toggle.el so both consumers reduce to thin delegates that pass their state-var symbols and defaults. The state vars stay where they were, so existing tests against each consumer's helpers keep working.

10 new tests cover the parameterized helpers in isolation. All consumer tests still pass.

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