fix(tools): StubProbe missing capabilities property breaks pyocd list --targets --pack=... (closes #1959)

[seam0814]

Summary

`DebugProbe.capabilities` is declared as an abstract property that
raises `NotImplementedError` in the base class. `StubProbe` —
instantiated by `ListGenerator.list_targets()` whenever a CMSIS-Pack
file is passed via `--pack=...` so that targets can be enumerated
without a real probe — did not override it.

The result is that any invocation of

```
pyocd list --targets --pack=<any_pack_file>
```

fails silently with a stripped error line and no traceback:

```
0000626 C Error: [main]
```

making the failure essentially undiagnosable. Reported in #1959 with
the root cause already pinpointed.

Closes #1959.

Change

```diff
class StubProbe(DebugProbe):
@Property
def unique_id(self) -> str:
return "0"
+

• @Property
• def capabilities(self):

•    return set()
  

```

An empty set is the truthful stub value — the placeholder probe has
no real features — and matches the expected `Set[Capability]` type
declared by the base class, so any downstream
`Capability.X in probe.capabilities` membership check now behaves as
"probe lacks every feature" rather than crashing.

Test

Added `test/unit/test_stub_probe.py` with two regression cases:

• `capabilities` returns `set()` (the contract this PR establishes)
• the returned value is iterable + membership-testable so downstream
  consumers don't change behaviour

```
$ python -m pytest test/unit/
1097 passed, 41 skipped in 1.87s
```

Scope

Two-file change. No public API change. `StubProbe` is only used
internally by `ListGenerator` for offline pack inspection; no
behavioural change to real probes (`CMSISDAPv1Probe`,
`CMSISDAPv2Probe`, `JLinkProbe`, etc.) all of which already provide
their own `capabilities`.