fix(keys): reject key chords with empty segments around +

[nityanand123gupta]

Problem

parseKeyChord filtered out empty segments after splitting a chord on +, so malformed bindings were silently reinterpreted as different, real shortcuts:

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

Fix

• Recognize the literal +/+ chord before segment validation (since splitting it produces two empty segments that must stay valid).
• For every other chord, reject any empty segment around + instead of filtering it out.
• Whitespace-padded valid chords (ctrl + s) continue to parse as before.

Testing

Ran locally (not just written blind):

• bun test src/extension-api/keys.test.ts src/ui/lib/keymap.test.ts — 36/36 passing, including new coverage for all four malformed cases, the literal-plus compatibility cases, and the keymap-level "malformed chord doesn't claim the real shortcut" case.
• bun run typecheck — clean.
• bun run lint — 0 warnings/errors.
• bun run format:check on the changed files — clean (the repo-wide format:check currently reports ~1000 pre-existing unrelated files; scoped to my 4 changed files only).

Added a patch Changeset per the contribution convention.

Fixes #906

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[nityanand123gupta]

Hi! The PR is currently waiting for workflow approval and Vercel authorization. Could a maintainer please approve the pending workflows when convenient? Thanks!