[Feature Request] gem4gov: add a license-assignment command (batchUpdateUserLicenses)

[JohnHales]

Feature Description

The gem4gov license group wraps distributeLicenseConfig (distribute) and lists configs (list), but has no user-assignment command. Add gem4gov license assign (plus unassign/reconcile) wrapping the public Discovery Engine batchUpdateUserLicenses REST method, mirroring the existing distribute command's structure.

Use Case

Assigning a license to an individual user currently requires the Gemini Enterprise console "Manage users" page or a manually created REST call (grep -n batchUpdateUserLicenses gem4gov.py → no matches). A CLI command makes assignment scriptable, idempotent, and auditable — producing cleaner audit evidence for license oversight.

Proposed Solution

Add gem4gov license assign wrapping the documented API. Verified schema (Google's public "Get subscriptions and assign licenses" docs):
POST https://{us|eu|}discoveryengine.googleapis.com/v1/projects/{PROJECT_ID}/locations/{LOCATION}/userStores/default_user_store:batchUpdateUserLicenses
body { inlineSource: { userLicenses: [{ userPrincipal, licenseConfig }], updateMask: { paths: ["userPrincipal","licenseConfig"] } }, deleteUnassignedUserLicenses: bool }.
Note: v1 (not v1alpha), default_user_store, licenseConfig is the project-level config path. Unassign = omit licenseConfig + deleteUnassignedUserLicenses: true.

Compliance & Deployment Context

• Target Deployment Type(s):
  • US Region Restricted (e.g., Access Policy constraint)
  • FedRAMP Medium
  • FedRAMP High
  • DoD IL4
  • DoD IL5
  • All / General
• Relevant NIST 800-53r5 Controls: AC-2 (account management) — auditable, repeatable license assignment.

Reusability Check

Stellar Engine prioritizes reusability.

• I have checked if this functionality can be achieved by extending an existing module or blueprint. (Extends the existing license group; no wrapper exists anywhere in the repo.)
• I have verified that this does not duplicate existing functionality. (Checked all 61 open/closed issues; [Feature Request] Convert gem4gov-cli Python tool to Terraform #100 proposes converting the CLI to Terraform — if that lands, this ask translates to the TF implementation.)

Alternatives Considered

Console clicks (not scriptable/auditable at scale); hand-rolled REST calls (works, but every operator reinvents the request shape).

Additional Context

Verified at main @ 3728fc98. Related: #100 (CLI→Terraform conversion).