Accept menu selection on a single keypress in bundle show/info/open

[eval]

I wanted to check if the following idea warrants a PR.

Working with claude-code, I got used to just selecting the number of an option when being offered choices (i.e. no need for Return). This made the current behaviour of e.g. bundle show <ambiguous name> stand out as added friction,
as Return is currently always needed.

Current behaviour

When a name matches more than one gem, bundle show/info/open print a numbered menu (via Bundler::CLI::Common#ask_for_spec_from) and read a whole line:

❯ bundle show rspec-
1 : rspec-support
2 : rspec-core
...
10 : rspec-rails
11 : rspec-retry
0 : - exit -
> 2⏎

Proposal

Accept the choice on a single keypress as soon as it's unambiguous, i.e. when no larger option number has the typed digits as a prefix:

• ≤ 9 options → any digit selects immediately, no Return.
• ≥ 10 options → only digits that are a prefix of a larger number wait (e.g. 1 when 10/11 exist); everything else (2–9, 0) still fires instantly. Return resolves the prefix to what's typed so far (e.g.
  selecting 1 while 10 exists). Backspace edits, Ctrl-C aborts.

It's a small, self-contained change in ask_for_spec_from, so all three commands (show/info/open) benefit since they share select_spec. Non-interactive use (pipes/CI, or when io/console is unavailable)
transparently falls back to the current line-based prompt, so existing behaviour and specs are preserved.

Note: input is restricted to valid digits

On the interactive path this also tightens input handling. Today the prompt reads a line and runs .to_i, so a typo like abc becomes 0 and exits, and 2x becomes 2. With single-keypress selection, non-digit keys (and digits that can't form a valid option number) are simply ignored, so a stray keystroke can no longer exit or mis-select. 0 still exits and Ctrl-C still aborts. The non-interactive fallback retains the current .to_i behaviour, so this stricter validation applies only to the live TTY prompt.

Open questions

1. Opt-out? Raw single-keypress input can surprise some terminals / screen-readers / automation. Should this sit behind a config setting (e.g. BUNDLE_SINGLE_KEYPRESS_SELECT, default on), or just be the default
   with the non-TTY fallback as the escape hatch?
2. Relation to bundle info is not using exact match #9441 / Implement --exact-match Flag for bundle info / show / open #9489. Those propose --exact-match to avoid the menu entirely; this proposal instead makes the menu faster when it does appear. They seem complementary rather than competing - does that
   match your thinking?

I have a working implementation with unit specs (bin/rspec spec/bundler/cli_common_spec.rb green, rubocop clean) and can open a PR if this seems worthwhile.