fix(extension-api): reject key chords with empty + segments

[Rehan30g]

Problem

The shared key-chord parser (parseKeyChord in src/extension-api/keys.ts) split a chord on + and filtered out any empty tokens before validating modifier/base-key structure. That let malformed bindings silently resolve to a different, valid shortcut:

s+       → s
+s       → s
ctrl++s  → ctrl+s
ctrl+s+  → ctrl+s

parseKeyChord("s+") returned the same parsed chord as parseKeyChord("s"), so a typo in a user [keybindings] entry, an extension registerCommand binding, or the public matchesKey helper could claim and fire a real shortcut instead of being rejected. This was inconsistent with the parser's existing behavior for other malformed chords like ctlr+s, f13, and ctrl+, which are already rejected.

Fix

Recognize the literal + chord (optionally whitespace-padded, e.g. " + ") before empty-segment rejection, since splitting it on + produces two empty segments that are intentional. For every other chord, split on +, trim each segment, and return a parse error if any segment is empty instead of silently dropping it. Whitespace-padded valid chords such as ctrl + s continue to parse.

The fix lives entirely in the shared grammar in src/extension-api/keys.ts, so built-in commands, user keybindings, extension registerCommand bindings, and the public matchesKey helper all inherit the stricter validation from one place.

Tests

• src/extension-api/keys.test.ts: added table-driven coverage asserting s+, +s, ctrl++s, and ctrl+s+ are rejected, and that +, " + ", and ctrl + s still parse correctly.
• src/ui/lib/keymap.test.ts: added a case confirming a malformed chord with an empty + segment (s+) does not claim another command's real shortcut (s) and is reported as one keybinding issue while its valid sibling chord survives.

Validation

bun test src/extension-api/keys.test.ts src/ui/lib/keymap.test.ts   # 36 pass
bun run typecheck                                                    # pass
bun run lint                                                         # 0 warnings, 0 errors
bun run format:check                                                 # pass
bun run deps:check                                                   # no violations
bun run test                                                         # 3364 pass, 35 skip, 2 fail (pre-existing, unrelated: a jujutsu test that requires the `jj` binary on PATH, and a React-internal error in an unrelated component test — both reproduce identically on main without this change)

A patch Changeset for hunkdiff is included.

Fixes #906

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