feat: Support Target TagSets in Tentacle Manager Registration

[HuyPhanNguyen]

Background

Octopus Server now supports Target TagSets (tagsets with "Target" scope) as a structured alternative to free-text roles. The web UI already supports selecting tags from these tagsets when registering deployment targets. However, the Tentacle Manager (desktop application) still only supports the legacy free-text roles dropdown, creating an inconsistent experience.

When users have configured Target TagSets in their server, they need to:

1. See all available tags from Target TagSets in the roles dropdown
2. Be prevented from selecting multiple tags from SingleSelect tagsets (which would cause validation errors on the server)

Results

[sc-132745]
The Tentacle Manager Registration wizard now:

• Loads target tagsets from the server when space data is loaded
• Adds target tagset tags to the roles dropdown alongside traditional roles
• Enforces SingleSelect constraints - when a user tries to select a second tag from a SingleSelect tagset, it's immediately removed with a clear error message
• Allows MultiSelect tags - multiple tags from MultiSelect tagsets can be selected freely

Before

• User can accidentally select multiple tags from a SingleSelect tagset, causing validation errors

After

Same familiar UI, but with intelligent validation:

• All target tagset tags appear in the roles dropdown
• Selecting a second tag from a SingleSelect tagset is immediately blocked with a helpful error message

How to review this PR

Quality ✔️

Pre-requisites

• I have read How we use GitHub Issues for help deciding when and where it's appropriate to make an issue.
• I have considered informing or consulting the right people, according to the ownership map.
• I have considered appropriate testing for my change.

[hnrkndrssn]

Code looks good to me, I'm unable to test it locally as it's Windows only but trust you have done due diligence testing on this change 👍

[acodrington]

This looks good, thanks!

I have some questions about tags/tagsets/roles work in general, but that doesn't need to block this code - I'll ask in Slack.