ddsidl: fix dead Python-keyword check in getAllowedName

[SoundMatt]

Problem

getAllowedName in src/vss_tools/exporters/ddsidl.py is meant to
prefix reserved-word identifiers (C, IDL, or Python) with a leading
underscore so the generated IDL is valid. The Python branch of the
check was dead:

if name.lower() in c_keywords or name.lower() in idl_keywords or keyword.iskeyword(name.lower):
                                                                                   ^^^^^^^^^^
                                                                            missing call parens

name.lower (without ()) is the bound-method object, not the
lowered string. keyword.iskeyword(<method object>) always returns
False, so VSS signal names that are Python keywords but NOT also
C/IDL keywords slipped through silently. Examples that bypass the
check: class, def, import, lambda, try, with, yield,
async, await.

The downstream effect is that the IDL exporter emits these names
unprefixed, producing syntactically invalid IDL.

Fix

Extract name.lower() to a local once and pass it explicitly to
keyword.iskeyword:

def getAllowedName(name):
    lower = name.lower()
    if lower in c_keywords or lower in idl_keywords or keyword.iskeyword(lower):
        return "_" + name
    else:
        return name

Behaviour-preserving for all inputs the current code handles
correctly; corrects the dead Python-keyword arm.

Test

Adds tests/test_ddsidl_naming.py with a regression test that
asserts getAllowedName("class") == "_class" (and similar for
def, import, lambda). With the original bug, that test fails.
Also covers normal names, IDL keywords, and C keywords to lock in
current behaviour.

Notes

• Only getAllowedName and the new test file are touched. No other
  call sites or behaviour changed.
• The redundant name.lower() call (three times in the original) is
  now called once, which is incidentally cleaner but the primary
  motivation is correctness.

[erikbosch]

LGTM

[erikbosch]

Thanks for the PR. I think it looks good but will leave it open for a while to give others a chance to review/comment if they would like to.

[sschleemilch]

remove the verbose explanation of the past

[erikbosch]

MoM:

• Presented at meeting
• ok to merge after maintainer approval

[erikbosch]

@SoundMatt - will you take a look at the comments from Sebastian

[SoundMatt]

Addressed — verbose comments removed.

[erikbosch]

@sschleemilch - do you want to take second look