Fix ghe.com source handling in migrate-repo

[brianaj]

Fixes #1604

What

Two small fixes for migrations that use a GitHub Enterprise Cloud with data residency (ghe.com) source via --github-source-api-url:

1. Migration source URL — the created migration source now uses the tenant URL (e.g. https://<tenant>.ghe.com) instead of always https://github.com. This is cosmetic and did not affect migrations.
2. Storage warning — the GitHub Enterprise Server (GHES) Management Console warning no longer shows for ghe.com sources (they have no Management Console). When --use-github-storage is passed, the CLI now notes the flag isn't required for ghe.com sources unless you are supplying your own archives.

Why

Reported in #1604: the source URL was hardcoded to github.com, and the GHES-specific storage warning surfaced on a source where it doesn't apply.

Changes

• Added a shared ExtractGitHubBaseUrl string extension that handles both GHES (.../api/v3) and ghe.com (api.<host>) URL shapes; replaced the private helper in the migrate-repo handler.
• CreateGhecMigrationSource now accepts an optional source URL (via an overload); the handler passes the tenant base URL for ghe.com sources.
• Gated the GHES Management Console warning and added a ghe.com-specific advisory.

Testing

• Unit tests added/updated for the extension, CreateGhecMigrationSource, and the migrate-repo handler. Full suite passing.
• Verified end-to-end against a ghe.com source, with and without --use-github-storage: the migration source uses the tenant URL and the storage warning behaves correctly in both cases.

Release notes

Added two entries to RELEASENOTES.md.

• Did you write/update appropriate tests
• Release notes updated (if appropriate)
• Appropriate logging output
• Issue linked
• Docs updated (or issue created)
• New package licenses are added to ThirdPartyNotices.txt (if applicable)

[Copilot]

Pull request overview

Fixes ghe.com source URL registration and storage-warning behavior in migrate-repo.

Changes:

• Extracts tenant base URLs from GHES/ghe.com API URLs.
• Registers ghe.com migration sources with their tenant URL.
• Adds source-specific storage guidance and tests.

Show a summary per file

File	Description
RELEASENOTES.md	Documents user-facing fixes.
src/Octoshift/Extensions/StringExtensions.cs	Adds base URL extraction.
src/Octoshift/Services/GithubApi.cs	Supports custom migration source URLs.
src/gei/Commands/MigrateRepo/MigrateRepoCommandHandler.cs	Applies tenant URLs and source-specific warnings.
src/OctoshiftCLI.Tests/StringExtensionsTests.cs	Tests URL extraction.
src/OctoshiftCLI.Tests/Octoshift/Services/GithubApiTests.cs	Tests custom source URL payloads.
src/OctoshiftCLI.Tests/gei/Commands/MigrateRepo/MigrateRepoCommandHandlerTests.cs	Tests ghe.com handler behavior.

Review details

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Suppressed comments (1)

src/gei/Commands/MigrateRepo/MigrateRepoCommandHandler.cs:437

• This guidance is inaccurate for archive URLs: those URLs are already hosted, and argument validation rejects --use-github-storage with only --git-archive-url/--metadata-archive-url (MigrateRepoCommandArgs.cs:98-107). The flag is applicable to on-disk archive paths, not archive URLs. Please remove that case from the warning.

                _log.LogWarning("The --use-github-storage flag is not required when migrating from GitHub Enterprise Cloud with data residency (ghe.com); the migration API handles storage. Only pass it if you are supplying your own archives via archive URLs or on-disk archive paths.");

• Files reviewed: 7/7 changed files
• Comments generated: 3
• Review effort level: Balanced

[github-actions]

Unit Test Results

    1 files      1 suites   25s ⏱️
1 163 tests 1 163 ✅ 0 💤 0 ❌
1 164 runs  1 164 ✅ 0 💤 0 ❌

Results for commit bc569e5.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
gei	81%	74%	688	✔
gl2gh	77%	70%	417	✔
bbs2gh	83%	78%	668	✔
ado2gh	71%	70%	749	➖
Octoshift	80%	70%	2000	✔
Summary	79% (8912 / 11290)	72% (2284 / 3177)	4522	✔