bug: acceptSuggestion bound to "return" breaks follow-up completions

[masahosono]

Describe the bug

When acceptSuggestion is bound to "return" via rc.toml, the first-level
completion is accepted correctly, but subsequent completions within the same
command (e.g. deeper path segments for cd) are never re-suggested.

The config is silently accepted with no warning, and first-level behavior
appears to work, so the failure is easy to miss until a user tries to chain
path completions.

To Reproduce

1. Create ~/.config/inshellisense/rc.toml: ```toml
   [bindings.acceptSuggestion]
   key = "return"
   shift = false
   ctrl = false
2. Start a session: is
3. Type cd and wait for directory suggestions.
4. Press Enter to accept a suggestion that ends with / (e.g. cd foo/).
5. Observe that no next-level suggestion is shown for the contents of foo/.

For comparison, with key = "tab" (default) the second- and deeper-level
completions are suggested automatically after each accept.

Expected behavior

Binding acceptSuggestion to "return" should behave equivalently to "tab"
for the completion flow: after accepting a suggestion, subsequent suggestions
should continue to appear as the command is extended.

Alternatively, if "return" is intentionally unsupported for this binding,
the config loader should reject it (or emit a warning) rather than accepting
it silently and producing broken behavior.

Environment

• OS: macOS 26.3.1 (Darwin 25.3.0)
• Output of is --version: 0.0.1
• Nodejs Version: 20.9.0
• Shell: zsh
• Shell Version: zsh 5.9