fix(agentless): improve session lock reliability

[krutibaraiya]

Background

We were getting the below errors when we set Consul Http address to a direct Server IP address or an ALB address in the Consul-ESM config file.

[INFO]  consul-esm: Agent: Trying to obtain health check session...
[INFO]  consul-esm: Trying to obtain leadership..
[ERROR] consul-esm: Agent: Unable to use session lock that was held previously and presumed lost, giving up the lock (will retry): error="Lock already held"
[INFO]  consul-esm: Agent: Trying to obtain health check session...
[ERROR] consul-esm: Unable to use leader lock that was held previously and presumed lost, giving up the lock (will retry): error="Lock already held"
[INFO]  consul-esm: Trying to obtain leadership...
[WARN]  consul-esm: Agent: Lost the lock
[WARN]  consul-esm: [WARN] Error querying for health check info: error="Get \"http://10.0.3.76:8500/v1/health/service/consul-esm?dc=dc1&index=1537773&partition=default&passing=1&tag=consul-esm\": context canceled"
[WARN]  consul-esm: Error getting external node list: error="Get \"http://10.0.3.76:8500/v1/catalog/nodes?dc=dc1&index=1444910&node-meta=external-node%3Atrue&partition=default\": context canceled"
[WARN]  consul-esm: Lost leadership

Fix

• Set SessionTTL to 30s and MonitorRetries to 6 for agentless session locks to handle transient Consul/network issues.
• Ensure lock is properly unlocked and reset to nil on loss, preventing "Lock already held" errors and enabling clean lock reacquisition.

Testing

Tested the binary on 1 ESM instance 3 Consul server setup, got only 3 errors in a span of 3 days

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.

• If applicable, I've documented the impact of any changes to security controls.

  Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.

[santoshpulluri]

Please add elaborate comments to describe the rationale behind setting them

    // sessionTTL is the TTL for Consul sessions created by ESM instances.
	// The session is automatically invalidated by Consul if it is not renewed
	// within this period. Must be longer than the total monitor window
	// (sessionMonitorRetries × DefaultMonitorRetryTime) to avoid split-brain,
	// where a new leader acquires the lock while the old session is still valid.
	sessionTTL = "30s"

	// sessionMonitorRetries is the number of consecutive failed attempts to
	// contact the Consul servers before the lock is considered lost and the
	// session is released. Combined with the default MonitorRetryTime of 2s,
	// this gives a monitor window of 6 × 2s = 12s — safely under the 30s
	// session TTL, ensuring the session expires before a new leader can
	// acquire the lock if Consul is truly unreachable.
	sessionMonitorRetries = 6

[santoshpulluri]

Changes looks good for me except for a minor comment about comment enhancements

[santoshpulluri]

LGTM