fix: [sc-85690] Auto updater integration test

[mlataza]

Summary

Story details: https://app.shortcut.com/rewst/story/85690

• Adds an end-to-end auto updater integration test across all three platforms (Windows, Linux, macOS)
• Fixes several reliability issues in the auto update flow: missing executable permissions, detached process launch, and an overridable update interval for testing

Changes

Auto updater integration test (scripts/build_integration_test.ps1, .github/workflows/integration-test.yml)

• Added scripts/build_integration_test.ps1 — builds a special integration test binary using -ldflags to inject a fake version (v0.0.0-it, older than any real release) and a short update interval (30s) to force an auto update during the test
• Added a build-integration-test job in the CI workflow that builds and caches the IT binary per platform using actions/cache@v5 (keyed on Go version + source hash)
• Added three new steps in each platform's test job (before the logging level update):
  1. Install the IT binary (old version) via --update
  2. Poll the log file until "Updating agent" appears (up to 120s)
  3. Verify the agent restarted on a newer version than 0.0.0-it

Update interval override (internal/agent/updater.go)

• Added updateIntervalStr package-level variable, injectable at build time via -ldflags
• Added DefaultUpdateInterval() which reads updateIntervalStr if set, otherwise defaults to 48 hours
• service.go updated to use DefaultUpdateInterval() instead of a hardcoded 48*time.Hour

Executable permission fix (internal/agent/updater.go)

• Added os.Chmod(file.Name(), 0755) after downloading the installer binary to os.CreateTemp — without this, exec fails with permission denied on Unix since temp files are created without execute bits

Detached process launch (cmd/agent_smith/run_command_unix.go, run_command_windows.go)

• The downloaded installer is launched via cmd.Start() (non-blocking), but it was inheriting the service's process group — causing it to be killed when the service shut down as part of the update handoff
• Added platform-specific detachedCommand():
  • Unix (Darwin/Linux): syscall.SysProcAttr{Setsid: true} — creates a new session, fully detaching from the parent process group
  • Windows: CreationFlags: syscall.CREATE_NEW_PROCESS_GROUP — equivalent detachment for the Windows process model
• Installer stdout/stderr are routed to the service's log file for visibility

Test plan

• Trigger the integration test workflow with os: all
• Verify the build-integration-test job completes and artifacts are uploaded for all three platforms
• Verify the "Wait for auto updater" step detects "Updating agent" within 120s on each platform
• Verify the "Verify auto update completed" step confirms the agent restarted on a version newer than 0.0.0-it
• Verify subsequent test steps (debug logging, syslog, uninstall) complete successfully after the auto update