fix: bound preOnline timeout so RTT-sensitive agents are not kicked offline

[om7057]

Fixes #1285

preOnline() waited on the agent's OpenTelemetry SDK configuration RPC with an unbounded Future.get(), while the sibling afterConfiguration() method already bounds the same kind of call to 10 seconds. Since preOnline() runs synchronously inside SlaveComputer.setChannel(), blocking there for an unbounded amount of time gives unrelated NAT idle timeouts or network disturbances a much bigger window to close the channel before the online handshake finishes. This mostly affects agents connecting over higher RTT links, where the configuration RPC naturally takes longer to complete.

This change bounds the wait to a configurable timeout, defaulting to 10 seconds to match afterConfiguration(), and logs a warning instead of blocking indefinitely when it elapses. The agent is still allowed online even if its OpenTelemetry configuration RPC has not completed in time.

The timeout is configurable via otel.instrumentation.jenkins.agent.pre_online.timeout (ConfigurationKey.OTEL_INSTRUMENTATION_JENKINS_AGENT_PRE_ONLINE_TIMEOUT), read the same way other OTel SDK properties are, through ConfigProperties.getDuration(...).

Testing done

Added OpenTelemetryConfigurerComputerListenerTest with two tests:

• preOnlineReturnsPromptlyWhenAgentConfigurationRpcNeverCompletes: uses a hudson.remoting.Future that never completes to simulate a slow or RTT-bound channel, and asserts preOnline() returns well within a few seconds once the configured timeout elapses, instead of blocking indefinitely.
• preOnlineDoesNotPropagateExecutionExceptionFromAgent: asserts a failed configuration RPC does not propagate out of preOnline().

Ran the full existing test suite locally on JDK 21 (matching this repo's Jenkinsfile): 238 tests run, 0 failures, 0 errors, 1 skipped (pre-existing skip, unrelated to this change). Also verified ./mvnw spotless:check and ./mvnw spotbugs:check pass clean.

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