Fix Linux extension exits 0 without writing status file on enable

[srikrishnaveturi]

Three related defects caused the Linux GPA extension to exit 0 without writing a status file on enable, making waagent read back its own pre-created placeholder (" \n"), fail to parse it as JSON, and report VMExtensionProvisioningError to CRP.

Fix 1 -- proxy_agent_shared/src/service.rs:
query_service_executable_path now returns Result instead of silently swallowing Err and returning an empty PathBuf. All callers are updated.

Fix 2 -- proxy_agent_extension/src/handler_main.rs:
enable_handler on Linux now always writes a Transitioning status for the current seq_no before exiting, regardless of whether the seq_no changed. Previously the write was guarded by should_report_status, so on VM restart with the same seq_no no status was written and waagent read back its own placeholder.

Fix 3 -- proxy_agent_extension/src/common.rs:
On Linux, get_proxy_agent_service_path() now returns the well-known fixed path (/usr/sbin/azure-proxy-agent) directly instead of querying systemctl show --property=ExecStart. The systemd unit-file cache is not hot right after install, causing ExecStart to return empty, which previously triggered a false VersionMismatch and a spurious re-install on every enable invocation.