False negative: narrow argument-shape checks in B508/B509

Describe the bug

B508 and B509 currently rely on narrow argument-shape checks in bandit/plugins/snmp_security_check.py.
This causes:

• false negatives for insecure SNMP usage when arguments are omitted/positional
• false positives for secure UsmUserData configs passed via kwargs

Reproduction steps

# Orignal Test Cases:

# SHOULD FAIL
a = CommunityData('public', mpModel=0)
# SHOULD FAIL
insecure = UsmUserData("securityName")
# SHOULD FAIL
auth_no_priv = UsmUserData("securityName","authName")
# SHOULD PASS
less_insecure = UsmUserData("securityName","authName","privName")

---
# False negatives cases:
CommunityData("public")                  # default mpModel=1 (v2c)
CommunityData("public", 1)               # positional mpModel
UsmUserData("securityName", "authName", None)  # authNoPriv

---
# False positive (should pass)
UsmUserData(userName="user", authKey="auth", privKey="priv")  # authPriv

Expected behavior

• B508 should treat omitted/positional/keyword mpModel consistently.
• B509 should evaluate privacy presence semantically (privKey / equivalent), not only positional arg count.

Bandit version

1.9.1 (Default)

Python version

3.14 (Default)

Additional context

No response

Start in bandit/plugins/snmp_security_check.py at the B508 and B509 checks, then compare their argument handling with each reproduction case in the issue. Use the omitted, positional, and keyword examples as regression cases; done means B508 consistently evaluates mpModel and B509 evaluates privacy semantically without the reported false negatives or false positive.