REV-1625 C5: Billing & Usage free-state

[jefflloyd]

Description

Implements the Billing & Usage free-state (REV-1625 work item C5, TECH spec §5 / P16).

When the user is in the plan-gated free state (FREE_AI_REMOVAL arm + Free plan + zero base allotment, via C4's AIRequestUsageModel::is_free_plan_ai_gated):

• The Balance section's base-credits meter card is replaced with a "Warp AI" card stating "The Free plan includes no Warp AI usage." — never a "0 of N" meter. The gated card also wins over a stale non-zero cached limit (e.g. when the FREE_PLAN_NO_AI denial arrives before the usage refresh).
• Remaining bonus-grant balances (personal/team cards) continue to render unchanged, so promo/referral credits stay visible until exhausted (P4/P17).
• A new actions row offers "Set up your own API key" (deep-links to AI settings via the existing NavigateToByokSettings action) and "Upgrade plan" (existing upgrade flow).
• Non-gated users (paid plans, arm-less Free users) see the page exactly as before (P25/P26).

Stacked on C4 (jeff/rev-1625-c4-gated-states); top of the REV-1625 stack (C3 → C4 → C5).

Linked Issue

• Linear: https://linear.app/warpdotdev/issue/REV-1625
• Spec PR: https://github.com/warpdotdev/warp-server/pull/11840

Testing

• cargo check -p warp, cargo clippy -p warp --tests (clean), ./script/format
• cargo nextest run -p warp -E 'test(billing_and_usage_page_v2)' — new unit test for the entitlement-card choice (meter vs gated card vs none), including the stale-limit case.

Local testing

1. Run against a local warp-server with S1+S3 (PRs Improve edit_files diff match failure errors #11841, Reorder tools panel to put project explorer first, conversation list second #11843):

   # warp-server: ./script/server
   # enroll a Free test user in the experiment arm (id 102):
   psql warp_development -c "INSERT INTO experiment_arm_user_overrides (experiment_arm_id, user_id) SELECT 102, id FROM users WHERE email = '<your-test-email>';"
   # this repo:
   cargo run --features with_local_server
   

2. Open Settings → Billing & Usage as the enrolled Free user: the Balance section shows the "Warp AI — The Free plan includes no Warp AI usage." card (no 0-of-N meter) plus the BYOK/upgrade buttons; "Set up your own API key" lands on Settings → AI with the API-key search applied; "Upgrade plan" opens the upgrade URL.
3. Grant the user bonus credits (INSERT INTO bonus_credit_grants ...) and refresh (↻ button in the Plan row): a Personal credits card with the remaining balance renders next to the gated card.
4. Delete the override row and refresh: the page returns to today's rendering (Balance section hidden for a 0-limit user without grants; meter card for limit > 0).
5. Paid account sanity: Build/Max/Enterprise accounts render the base-credits meter exactly as before.

Agent Mode

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

Conversation: https://staging.warp.dev/conversation/b0196734-7ef9-43b3-acc2-e243d9e053d7
Run: https://oz.staging.warp.dev/runs/019eb89c-4bd7-75cd-8c8a-398dabd0afde

This PR was generated with Oz.