fix: round relative time seconds exactly

[hiSandog]

Motivation

RelativeTimeUtil.nsToSeconds currently builds a BigDecimal from a double. For half-up rounding cases such as 1.2345 seconds, the binary double value can be slightly below the decimal half and round down unexpectedly.

Modifications

• Convert nanoseconds with BigDecimal.valueOf(ns, 9) so the decimal value is exact before rounding.
• Apply the same fix to both common and cli-utils RelativeTimeUtil implementations.
• Add coverage for a half-up rounding boundary.

Verifications

• git diff --check
• Not run: ./gradlew :pulsar-common:test --tests "org.apache.pulsar.common.util.RelativeTimeUtilTest" -PtestRetryCount=0 (local environment has no Java Runtime)