fix: remove dead sessions from target session list

[carmark]

Summary

Fixes #42

When an iSCSI connection closes or a logout request is received, the associated session was never removed from the target's Sessions map, the ITNexus was never cleaned up, and the TSIH was never released. This caused resource leaks on every connect/disconnect cycle.

Root Cause Analysis

UnBindISCSISession() was only called from ReInstatement() (session replacement). It was never called from:

• handler() when connection enters CONN_STATE_CLOSE (normal disconnect, error, etc.)
• iscsiExecLogout() when initiator sends a logout request

Additionally, ReleaseTSIH() existed but was never called anywhere, so TSIHs were allocated but never freed.

Impact

• Dead sessions accumulate in target.Sessions map
• ITNexus entries leak in target.ITNexus map
• TSIH bitmap slowly exhausts (max 65534 sessions before total exhaustion)
• Target deletion may fail if orphan sessions exist

Changes

pkg/port/iscsit/session.go:

• UnBindISCSISession(): Now also calls ReleaseTSIH() to free the TSIH
• New removeConnectionFromSession(): Removes a connection from its session; if no connections remain, unbinds the session entirely. Uses conn.session = nil guard to prevent double-cleanup.

pkg/port/iscsit/iscsid.go:

• handler(): Calls removeConnectionFromSession(conn) before closing the TCP connection on CONN_STATE_CLOSE
• iscsiExecLogout(): Converted from free function to ISCSITargetDriver method; now calls removeConnectionFromSession() to clean up session on logout

Test plan

• go build ./... passes
• go test ./pkg/... ./mock/... all pass
• CI workflow passes

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