Allow installing runtimes from a script shebang.

[zooba]

Fixes #235

[Copilot]

Pull request overview

This PR adds the ability to install Python runtimes based on the shebang line found in a script file. This addresses issue #235 by allowing the install command to read a script's shebang (e.g., #! python1.1.exe) and automatically install the corresponding runtime if it's not already present.

Key changes:

• Modified install_command.py to extract runtime specifications from script shebangs when the --from-script flag is used
• Replaced global DOWNLOAD_CACHE with a per-command cache stored in cmd.scratch for better testability
• Added test infrastructure and test cases to verify the new functionality

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
src/manage/install_command.py	Added logic to parse shebang from scripts and extract runtime tags; replaced global DOWNLOAD_CACHE with cmd.scratch-based caching
tests/test_install_command.py	Added InstallCommandTestCmd helper class and test cases for script-based installation
tests/conftest.py	Modified assert_log to perform type conversion when comparing logged values with expected values

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