fix: stabilize session date and connection test rate

[Kaguya-19]

Summary

• increase model connection-test limit from 5 to 20 requests per minute
• distinguish an in-flight test (TEST_BUSY) from a true request-rate limit (RATE_LIMITED)
• cap the entire multi-model connection-test batch at 60 seconds so stalled probes release the per-user slot
• keep busy and invalid retries from consuming the one-minute rate bucket
• return the actual remaining Retry-After for both per-user contention and a full global probe pool
• keep the onboarding button and error message on the same live countdown, then clear the retry message at zero
• prevent rapid repeated clicks from submitting duplicate connection tests
• remove the superseded session-date implementation now covered by merged PR fix(context): stabilize session prompt dates and append daily updates #571

Root cause

A multi-model test could run sequential 60-second probes without a batch timeout, holding the per-user concurrency slot for several minutes. Retries were mapped from TEST_BUSY to RATE_LIMITED and also counted against the minute bucket, so waiting one minute did not clear the apparent rate-limit error. The global probe limiter also lacked global start-time tracking, causing unrelated users to receive an unhelpful one-second retry delay when all slots were occupied.

Validation

• npm test -- server/routes/onboarding.test.js server/routes/config.test.js src/components/onboarding/view/subcomponents/LlmConfigurationStep.test.tsx (132 passed)
• global-pool regression coverage verifies the fourth user receives the earliest active probe slot as Retry-After
• retry countdown coverage verifies both the button and error message update each second and the message clears at zero
• regression coverage verifies 21 busy retries do not consume the rate bucket and the next request succeeds after the active test finishes
• node --import tsx --test --test-timeout 60000 tests/context/cache-runtime.spec.ts (4 passed after removing the superseded date test)
• targeted ESLint passes for the updated onboarding server files
• git diff --check passes