refactor!: guard install flow against dot-sourcing

[chawyehsu]

This COULD be a BREAKING CHANGE, as the behavior has changed.

This PR adjusts the execution prerequisites for the installer's core install flow. When the installer is imported via dot-sourcing (i.e., . .\install.ps1), the install flow will no longer run. This aligns with the intended behavior and distinguishes between simply importing functions defined in the installer via dot-sourcing, and directly executing the installer.

This change was prompted by a CI error in #135, tests should not execute the install flow.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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• install.ps1

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Summary by CodeRabbit

• Bug Fixes

  • Improved installer error handling so failures stop installation consistently while preserving the previous PowerShell error preference.
  • Prevented installation steps from running when the installer is loaded rather than executed directly.
  • Moved language-mode validation into the installer execution checks.
  • Improved prerequisite validation before applying proxy settings.

• Tests

  • Updated installer test setup to use isolated temporary directories for local and global installations.

Walkthrough

Changes

The installer checks prerequisites before parameter validation and temporarily uses terminating errors during direct execution. Tests assign Scoop paths directly from $TestDrive.

Changes

Installer behavior

Layer / File(s)	Summary
Installer execution flow install.ps1	Install-Scoop checks prerequisites before validating parameters. Direct execution sets $ErrorActionPreference to Stop for the install flow and restores the previous value afterward.
Test environment initialization test/install.Tests.ps1	Tests assign SCOOP and SCOOP_GLOBAL directly from Join-Path expressions under $TestDrive.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely identifies the main change: guarding the install flow against dot-sourcing.
Description check	✅ Passed	The description directly explains the dot-sourcing behavior change, its impact, and the related CI issue.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

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[coderabbitai]

Actionable comments posted: 1

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: eaa45aa4-9822-49df-8ad6-60492464dcd3

📥 Commits

Reviewing files that changed from the base of the PR and between 3f16afb and a26a560.

📒 Files selected for processing (2)

• install.ps1
• test/install.Tests.ps1

Included review availability: Your plan provides up to 4 included reviews per hour; 3 remain after this review.