Stop confirmation prompts from looping forever on stdin EOF

[mbrt]

askYN only accepted an answer when ReadString returned no error, and reprompted otherwise. Once stdin reaches EOF (e.g. gmailctl apply </dev/null, a background job or a CI runner), every subsequent read fails immediately with io.EOF, so the loop printed "invalid choice" and reprompted forever instead of exiting.

askOptions, used by gmailctl edit, had the same problem: fmt.Scanln keeps returning io.EOF, which was treated like any other invalid input.

Treat the end of input as the final answer:

• askYN defaults to "no", the same as an empty line, so apply exits without changing anything. An answer typed without a trailing newline (e.g. printf y | gmailctl apply) is still honored, since that is clearly what the user meant.
• askOptions returns an error, which edit turns into an abort. This leaves the original config untouched and prints where the edited copy was saved, exactly like choosing "abort". Choosing "no (continue editing)" instead would reopen the editor and hit the same EOF again.

In both cases a newline is printed, so that what follows doesn't end up on the prompt line.

The prompts now take their input and output streams as arguments, so this behavior is covered by unit tests.

Close #464.

[codecov]

Codecov Report

❌ Patch coverage is 78.12500% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 41.37%. Comparing base (f1f0aef) to head (abc3c43).

Files with missing lines	Patch %	Lines
cmd/gmailctl/cmd/edit_cmd.go	0.00%	6 Missing ⚠️
cmd/gmailctl/cmd/apply_cmd.go	0.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (78.12%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #467      +/-   ##
==========================================
+ Coverage   39.22%   41.37%   +2.14%     
==========================================
  Files          54       54              
  Lines        3681     3693      +12     
==========================================
+ Hits         1444     1528      +84     
+ Misses       2136     2063      -73     
- Partials      101      102       +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.