Propagate Vivado target query failures

[jeet-dekivadia]

Summary

• propagate a failed Vivado hardware-target query instead of treating it as an empty target list
• add a focused regression test for the non-zero exit path

Why

get_available_targets() currently parses stdout regardless of Vivado's exit status. When Vivado cannot connect to hw_server or the TCL script fails, the caller falls through to the generic "No hardware targets found" message and suggests checking JTAG wiring or board power. Propagating the subprocess failure preserves the distinction between an empty successful query and a failed query.

Validation

• git diff --check
• python3 -m compileall -q fpga/common/hw_target.py tests/test_hw_target.py
• python -m pytest tests/test_hw_target.py