[Repo Assist] test: add coverage for handle_normal_mode key bindings

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What

Adds 28 new tests to src/handler.rs covering handle_normal_mode key bindings that had no test coverage.

Why

handle_normal_mode is the central keyboard dispatcher — every normal-mode keystroke routes through it. Most of its branches (quit, help, focus, sort, install, uninstall, upgrade, pin, source filter, view switching, multi-select) were completely untested, meaning regressions could slip through silently.

Covered key paths

Keys	What is tested
q / Esc	should_quit flag is set
?	toggles show_help overlay on and off
Tab / BackTab	focus zone cycles between list and detail
S	sort cycles from None to Name ↑
p (Search mode)	shows informational status, no confirm
p (truncated ID)	shows "truncated" status, no confirm
p (unpinned pkg)	creates Pin confirm dialog
p (pinned pkg)	creates Unpin confirm dialog
P (Search mode)	shows informational status
P (Installed mode)	cycles pin_filter All → PinnedOnly
i (truncated)	shows "truncated" status, no confirm
i (valid pkg)	creates Install confirm dialog
x (truncated)	shows "truncated" status, no confirm
x (valid pkg)	creates Uninstall confirm dialog
u (truncated)	shows "truncated" status, no confirm
u (valid pkg)	creates Upgrade confirm dialog
I (truncated)	shows status, stays in Normal mode
I (available version)	prefills version_input from available_version
I (no available version)	falls back to version
a	selects all packages in Upgrades view
a (all selected)	deselects all
f	cycles source filter All → Winget
r	sets loading = true, status = "Refreshing..."
Right	cycles view forward (Search → Installed)
Left	cycles view backward (Installed → Search)
Space	toggles package selection; second press deselects

Test Status

cargo check --all-targets  → clean
cargo fmt -- --check       → clean
cargo clippy -- -D warnings → clean
cargo test                 → 260 passed (was 232) — 28 new tests

No production code changed.

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