Test-mode flat-100 MTU cap (separate set, never bills)

[axelgar]

What to build

Close the test-key bypass: today test mode short-circuits metering entirely (no cap beyond the per-project rate limit), making it a trivial production bypass. Add a small, separate test-MTU cap that reuses the blocked contract from #2.

• A separate test-MTU set (kept out of the billing meters — test usage must never appear on any invoice).
• Flat 100 distinct MTU/month ceiling, every tier (100 not 1k, so it can't cannibalise the live-key upgrade lever).
• Same new-user block semantics as the Free ceiling: existing test users always pass; a new user past 100 is refused with blocked: "bismite_test_limit"; counter-down → fail-open.
• Test keys stay per-project rate-limited as today.
• SDK already surfaces blocked from Free-ceiling new-user block, end to end (establishes the blocked contract) #2 — this slice just emits the test reason.

Acceptance criteria

• The 101st new test user this period is refused with blocked: "bismite_test_limit".
• Test usage never moves any billing meter / never appears on an invoice.
• Existing test users this period still pass; counter-down → fail-open.
• Per-project rate limit on test keys still applies.

Blocked by

• Free-ceiling new-user block, end to end (establishes the blocked contract) #2 (reuses the SDK blocked contract)