feat: Add Spring Boot Actuator Health Indicators for Hiero Services

## Problem

While exploring the Spring integration, I noticed that there are currently no built-in health checks for important Hiero dependencies such as the Mirror Node or the operator account balance.

In production environments, these are usually things teams want visibility into early before failures start affecting the application.

For example: if the configured Mirror Node becomes unreachable or if the operator account balance becomes too low for transactions. The application may continue running until requests start failing at runtime.

## Proposed Solution

Add Spring Boot Actuator health indicators for common Hiero-related checks so applications can expose this information through the standard `/actuator/health` endpoint.

Possible Health Indicators:
- `MirrorNodeHealthIndicator` : A lightweight check to verify connectivity with the configured Mirror Node REST API.

- `OperatorBalanceHealthIndicator`: Checks the current operator account balance and compares it against a configurable threshold.

For example: if the balance becomes critically low, the health endpoint could report DOWN or expose a warning in the health details

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## Why This Could Help

Spring Boot Actuator is already commonly used in enterprise applications for monitoring application health and external dependencies. Adding built-in Hiero health indicators would make it much easier for teams to monitor things like:

- Mirror Node availability
- operator account balance status

through existing monitoring setups such as Prometheus, Grafana, or Datadog.

It could also help applications detect operational issues earlier instead of only discovering them after transactions or queries start failing.

The issue names no files or tests. Start by reviewing the Spring Boot Actuator integration around the `/actuator/health` endpoint, then determine how Mirror Node connectivity and operator balance access are exposed. Done means both health indicators report meaningful status, and the balance check supports a configurable threshold.

Written by the indexing model from the issue text.