[fix][test] Stabilize WebService rate limiting test

[oneby-wang]

Motivation

WebServiceTest.testRateLimiting can be flaky because setup admin HTTP requests and lookup HTTP requests share the same broker HTTP rate limiter. With a 10 QPS limiter, requests sent too closely together can consume freshly restored permits and cause an unexpected 429 before the test reaches the burst section.

Modifications

• Compute the wait interval from rateLimit and use it for lookup pacing.
• Add the same short wait between setup admin calls and after namespace creation.

Verifying this change

• Make sure that the change passes the CI checks.

Does this pull request potentially affect one of the following parts:

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment