fix: prevent graceful shutdown errors from killing Envoy proxy immediately

[santoshpulluri]

Problem

During graceful shutdown, any error from DumpConfig(), Drain(), or Quit() calls close(errorExitCh). This channel is monitored by the Run() select loop in consul_dataplane.go:

case <-cdp.lifecycleConfig.lifecycleServerExited():
    // Calls Quit() + Kill() immediately

This means a transient error during shutdown (e.g., Envoy not yet ready, network timeout, IPv6 URL issue) bypasses the entire grace period and kills Envoy at 0ms — exactly the symptom seen in TestConnectInject_ProxyLifecycleShutdown acceptance test failures where curl reports Connection refused immediately.

Sequence of events (before fix)

SIGTERM → gracefulShutdown()
  → DumpConfig() fails (any reason)
  → close(errorExitCh)                    ← fires immediately
  → Run() select: lifecycleServerExited()
  → proxy.Quit() + proxy.Kill()           ← Envoy killed at 0ms
  → all connections refused instantly

Root cause

errorExitCh is intended to signal that the lifecycle server itself has crashed (e.g., ListenAndServe fails). It should NOT be closed for transient errors during an already-in-progress graceful shutdown. The shutdown path should log errors and continue — the grace period must be honored regardless.

Fix

Removed close(errorExitCh) from all three error paths in gracefulShutdown():

• DumpConfig() error
• Drain() error
• Quit() error

Errors are still logged as warnings. The shutdown continues through the full grace period as intended.

Testing

Added 3 unit tests in lifecycle_test.go that reproduce the bug deterministically:

Test	Verifies
TestGracefulShutdown_DumpConfigError_DoesNotKillProxy	DumpConfig() failure does not close errorExitCh
TestGracefulShutdown_DrainError_DoesNotKillProxy	Drain() failure does not close errorExitCh
TestGracefulShutdown_QuitError_DoesNotKillProxy	Quit() failure does not close errorExitCh

Also fixed pre-existing issues in the test mock:

• mockProxy methods now return error fields — Drain(), Quit(), DumpConfig() previously always returned nil, meaning error paths were never exercised
• Fixed data race — mock counters changed from int to sync/atomic.Int32 to eliminate races flagged by go test -race

Related PRs

• fix: add skip_exit to drain_listeners to prevent premature pod termin… #1064 — Added skip_exit to Drain() + JoinHostPort (prevents Envoy self-terminating on drain)
• fix: use net.JoinHostPort for remaining Envoy admin URLs #1066 — JoinHostPort for DumpConfig() + Ready() (prevents malformed IPv6 URLs)
• This PR — Defense-in-depth: even if admin endpoint calls fail for any reason, the grace period is still honored

All three are needed together to fully fix TestConnectInject_ProxyLifecycleShutdown.