Refactor input

[kmvanbrunt]

Moved logic specific to the main command line from Cmd.read_input() to Cmd._read_command_line().

No longer setting Cmd.session.input to a pipe since that causes warnings from prompt-toolkit.
For non-interactive sessions (self.stdin is a pipe), we use self.stdin.readline().

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 97.50000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.17%. Comparing base (4f545de) to head (a300f10).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd2/cmd2.py	97.43%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1585      +/-   ##
==========================================
- Coverage   99.42%   99.17%   -0.25%     
==========================================
  Files          21       20       -1     
  Lines        4676     4616      -60     
==========================================
- Hits         4649     4578      -71     
- Misses         27       38      +11     

Flag	Coverage Δ
unittests	99.17% <97.50%> (-0.25%)	⬇️

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