Fix Windows test exit codes not being checked

[Copilot]

This PR fixes an issue where test scripts with multiple commands on Windows would ignore failures in earlier commands, only checking the exit code of the final command. This behavior was inconsistent with conda-build and could lead to packages passing tests when they should fail.

Problem

When running test scripts on Windows with multiple commands like:

test:
  commands:
    - python --version
    - python -c "import nonexistent_module"  # This fails
    - echo Test completed successfully       # This still runs!

The test phase would succeed even if the import command failed, because only the last command's exit code was checked.

Solution

Added Windows-specific exit code checking to the CmdExeInterpreter that inserts IF %ERRORLEVEL% NEQ 0 EXIT 1 after each command (except the last), mimicking conda-build's proven approach:

python --version
IF %ERRORLEVEL% NEQ 0 EXIT 1
python -c "import nonexistent_module"
IF %ERRORLEVEL% NEQ 0 EXIT 1
echo Test completed successfully

Now if any command fails, the script immediately exits with error code 1.

Changes

• Core Fix: Added CmdExeInterpreter::add_exit_code_checks() method that intelligently processes Windows batch scripts
• Applied to both: CmdExeInterpreter::run() and Output::create_build_script() methods
• Smart filtering: Skips adding checks for comments (@rem, ::, etc.), labels (:label), and empty lines
• Platform-specific: Only affects Windows cmd.exe interpreter, no impact on bash/other interpreters
• Comprehensive testing: Added 11 unit tests covering various scenarios and edge cases

Verification

The fix has been thoroughly tested with scenarios including:

• Multiple commands with potential failures
• Scripts with comments and labels
• Empty lines and whitespace handling
• IF statements and ECHO commands
• Single command scripts (no regression)

All existing tests continue to pass, ensuring no breaking changes.

Fixes #1792.

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[wolfv]

What about line continuation with ^?

[Copilot]

Line continuation with ^ is already handled! The code detects lines ending with ^ (line 67), tracks continuation state (lines 82-88), and only adds exit code checks after the complete command finishes (lines 95-98). See tests test_add_exit_code_checks_with_line_continuation() and related tests at lines 276-310 for verification.