fix(install): use releases api in powershell installer

[Shengming Liang (LiangShengming)]

Pull Request

Summary

What:

Fix the Windows PowerShell installer latest-version lookup by reading tag_name from the GitHub Releases API instead of parsing /releases/latest redirect headers.

Why:

PowerShell 5.1 can misread the typed redirect response headers and fail before downloading the CLI. The API response gives us a stable JSON shape across supported PowerShell versions.

Test Plan

• git diff --check upstream/feat/self-update-command..HEAD
• ./scripts/test
• gh api repos/dedalus-labs/dedalus-cli/releases/latest --jq .tag_name -> v0.4.0
• GitHub Actions Windows smoke: https://github.com/dedalus-labs/dedalus-cli/actions/runs/28067321724
  • powershell 5.1 (windows-2022) passed with PSVersion 5.1.20348.4294, PSEdition Desktop, installer resolved v0.4.0, installed dedalus.exe, and dedalus.exe --help reported 0.4.0.
  • powershell 5.1 (windows-latest) passed with PSVersion 5.1.26100.32995, PSEdition Desktop, installer resolved v0.4.0, installed dedalus.exe, and dedalus.exe --help reported 0.4.0.

Repro / Showcase

For the installer bug, Get-LatestVersion no longer depends on the PowerShell 5.1 redirect Location header shape. It now calls https://api.github.com/repos/dedalus-labs/dedalus-cli/releases/latest with a User-Agent and reads tag_name from JSON.

Tests Added

• Unit tests
• Integration tests
• E2E tests
• N/A (no new code paths)

Risk areas:

• The installer now depends on GitHub's releases API rather than the web redirect endpoint.
• The unauthenticated API path is rate-limited by GitHub, but normal installer usage should be well below that limit.

[Shengming Liang (LiangShengming)]

Validation evidence for this PR at commit 4633827:

• Windows PowerShell 5.1 smoke passed in GitHub Actions: https://github.com/dedalus-labs/dedalus-cli/actions/runs/28067321724

  • windows-2022: C:\Windows\System32\WindowsPowerShell\v1.0\powershell.EXE, PSVersion 5.1.20348.4294, PSEdition Desktop
  • windows-latest: C:\Windows\System32\WindowsPowerShell\v1.0\powershell.EXE, PSVersion 5.1.26100.32995, PSEdition Desktop
  • Both resolved latest as v0.4.0, installed dedalus.exe, and dedalus.exe --help reported 0.4.0.

• Normal CI passed for the same commit: https://github.com/dedalus-labs/dedalus-cli/actions/runs/28067375030

  • lint: passed
  • test: passed
  • build: passed