[dgibbs64]

Description

When a configured LinuxGSM reference has been mistyped or deleted, report Branch "<branch>" not found on <user>/<repo> and exit before downloading or replacing files. Do not silently switch branches.

Validate through the GitHub contents API at the repository root, so a missing individual file is not mistaken for a missing reference. Tags, commit SHAs and branch names containing / remain supported. Check lazily before downloads and explicitly in update-lgsm, caching the result for the configured user/repository/reference. Cached commands retain their existing offline behaviour and upstream master downloads remain pinned to the installed version tag.

Only HTTP 404 is fatal. API rate limits, DNS failures, timeouts and server errors retain the existing GitHub/Bitbucket download fallback. Probes have a three-second connection timeout and ten-second total timeout. The GitHub API also returns 404 for unavailable repositories; the diagnostic identifies the configured repository alongside the reference.

New modules remain compatible with older launchers: they skip the helper until update-lgsm replaces the launcher, then validation applies on the next invocation. Failure exits directly because loading an exit module could itself require a download from the missing reference.

Fixes #4994

Type of change

• New feature (a change which adds functionality).

Testing

• python3 tests/test_branch_validation.py: 14 tests passed, including bootstrap and core downloads, missing references, once-per-reference probing, cached/forced downloads, 403/429/500/503/DNS/timeout fallback, update selection, upstream version pinning, forks, slash-containing branches, tags/SHAs, unrelated repositories and legacy launchers. HTTP responses are mocked; actual Bash function bodies and update repository selection are exercised.
• Live temporary-installation smoke tests with the complete launcher and real GitHub requests: list succeeded for develop, master, v26.2.0 and commit 2a494011fb636642b9a5d1332a79e359586c54f8. A nonexistent reference exited 1 with the expected diagnostic and no downloaded server list.
• bash -n and shellcheck passed for all three modified Bash files.
• Prettier checks passed for the changed Bash files and workflow; Black, Ruff, Pylint and mypy passed for the regression suite.
• Added a GitHub Actions workflow to run the regression suite on relevant pushes and pull requests.
• Environment: WSL distribution Ubuntu-24.04, running Ubuntu 26.04.1 LTS. No game server installation or live service was changed.

Risk and rollback

• Risk level: low. Adds one bounded API request when remote access is already needed; non-404 API failures do not prevent existing fallback.
• Rollback plan: revert this PR.

Breaking changes

• No breaking changes.

Documentation impact

• No documentation update required.

Checklist

• This pull request links to an issue.
• This pull request uses the develop branch as its base.
• This pull request subject follows the Conventional Commits standard.
• This code follows the style guidelines of this project.
• I have performed a self-review of my code.
• I have provided a detailed enough description of this PR.

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