fix(extension): preserve plus keys in keyboard chords - #308

Thanks for the focused fix! Validation against main (8ca7911) confirms that the issue still exists and that this patch fixes the intended Ctrl++ / Cmd++ cases.
There’s one small edge case I’d like to tighten before merging: the trailing-plus check currently also accepts prefixes containing unknown tokens or an existing base key. For example:

• Crtl++ (a typo) previously returned invalid_params, but now succeeds and inserts a literal + into the focused input.

• Ctrl+Bogus++ previously failed, but now dispatches Ctrl + +.

• Ctrl+A++ changes from the existing Ctrl+A interpretation to Ctrl + +.

Could we restrict the new plus-key handling to cases where the preceding tokens are all recognized modifiers? That would preserve the intended fix while avoiding these additional behavior changes.

Please also add regression coverage confirming that unknown-prefix cases return invalid_params without dispatching any key events, and that an existing base key is not overwritten by the new handling.

A small guard around the new branch should be sufficient here. With that adjustment and coverage, I’d be happy to move this toward merge.