feat: Add experiment run polling helper

[rowing0328]

Description

Add a polling helper that waits until an experiment run reaches a terminal phase.

The SDK currently exposes LitmusClient#getExperimentRun(), which retrieves the current state of an experiment run once.
Applications that need the final result must implement their own polling loop, timeout handling, and terminal-phase checks.

This feature should provide a consistent SDK-level helper for that workflow.

Proposed API

ExperimentRun waitForExperimentRun(
    String projectId,
    String experimentRunId,
    Duration timeout,
    Duration interval
)

Scope

• Add waitForExperimentRun() to src/main/java/io/litmuschaos/LitmusClient.java.
• Add a polling helper class under src/main/java/io/litmuschaos/util/.
• Reuse the existing getExperimentRun() query.
• Poll at the configured interval until a terminal phase is reached.
• Return the final ExperimentRun response when polling finishes successfully.
• Throw a clear exception when the configured timeout is exceeded.
• Add JavaDoc and unit tests.
• Document the behavior for failed, stopped, and error phases.

Acceptance Criteria

• Polling continues while an experiment run is in a non-terminal phase.
• Polling stops when a documented terminal phase is reached.
• The final ExperimentRun response is returned as documented.
• A clear exception is thrown when the timeout is exceeded.
• The polling interval and timeout are configurable with Duration.
• Unit tests cover terminal completion and timeout behavior.
• JavaDoc documents terminal and unsuccessful phase handling.