fix: tagset update remove all tags

[HuyPhanNguyen]

Problem:
Updating tagset properties (name, description, etc.) would delete all tags within that tagset.

Root Cause:
The Octopus API requires the complete Tags array in update requests. Missing tags are interpreted as deletions. The provider's expandTagSet() never populated the Tags field, causing all tags to be removed on every tagset update.

Solution:

• Fetch current tagset before updating to preserve existing tags
• Add mutex locking to tagset operations to prevent race conditions with concurrent tag updates
• Add test case verifying tags survive tagset updates

Fixes [sc-128359]