Gate native workspace team creation to admins

[warp-agent-staging]

Description

Native workspaces were enabled solely from billing policy, even when a workspace had fewer than two teams. The Teams create/join page also always exposed team creation, regardless of workspace-level permissions.

This change:

• requires an enabled native-workspaces policy and more than one team;
• hides the complete Create a team section for native-workspace non-admins or unresolved identities;
• preserves existing create-team behavior for non-native workspaces and native workspace admins;
• keeps team discovery visible with a standalone heading when creation is hidden.

Linked Issue

APP-5171

Originating thread: https://warpdev.slack.com/archives/C0BDQDW8V5E/p1785873369852859

Testing

• Regression tests intentionally omitted at the requester's explicit direction for this change.
• ./script/format --check
• CARGO_INCREMENTAL=0 cargo clippy -p warp --all-targets --tests -- -D warnings
• CARGO_INCREMENTAL=0 cargo build -p warp
• Manually exercised native-admin, native-non-admin, and non-native Settings > Teams states on the current PR head in the running authenticated Warp client using temporary runtime fixtures that were removed after capture.

Screenshots / Videos

Computer-use screenshots

Computer-use video recordings

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-IMPROVEMENT: Restrict team creation in native workspaces to workspace admins.

Conversation: https://staging.warp.dev/conversation/03ef05c7-2917-4f80-b77d-3a3c6441ead1
Run: https://oz.staging.warp.dev/runs/019fce62-eeb0-7385-8347-f710351b7c7f

Co-Authored-By: Warp Agent agent@warp.dev
This PR was generated with Oz.