Fix silent and crashing bugs in the netsnmp and netntp states

[ggiesen]

What does this PR do?

Fixes five bugs in the netsnmp and netntp NAPALM network-config states -- two
of them silent (report success while doing the wrong thing), which is why they
went unnoticed.

netsnmp:

1. _expand_config did defaults.update(config), crashing with
   AttributeError: 'NoneType' object has no attribute 'update' whenever the
   state declares no defaults (the common case) -- the same pattern as [BUG] Unable to add users on junos via napalm #62170's
   netusers bug.
2. _clear_community_details had community_details.get["mode"] = ..., a typo
   that subscripts the bound .get method and raises TypeError for every
   community given in the (documented, recommended) dict form.
3. _create_diff's third branch was elif not fun(curr), which is unreachable
   once the first two branches are past -- so a valid->valid change (e.g.
   location "OldTown" -> "NewTown") matched no branch, was dropped from the
   diff, and the state reported success without pushing the change.

netntp:
4. _check resolved domain names into ip_only_peers and then did
peers = ip_only_peers, rebinding a local -- the resolved addresses were
discarded, so domain-name peers never converged (the device reports IPs, the
desired list kept the names, so the diff never emptied). It now rewrites the
caller's list in place, and keeps an unresolvable entry (no DNS resolver
available) instead of silently dropping it.
5. managed masked a device-retrieval failure as success: a failed
ntp.peers/ntp.servers produced no changes, which fell through to the
"no changes -> Device configured properly" branch and returned result=True.
The retrieval-failure path now flags successfully_changed=False, and
managed returns result=False before that branch.

Validation on real hardware

Validated against a live Juniper EX3400 (Junos 23.4R2) over NETCONF, test=True
(which exercises the full diff path without touching the device templates):

• netsnmp.managed with community: {splab: {mode: read-only}} -> before: the
  AttributeError crash; after: normalizes the mode to ro and returns a
  correct diff.
• netntp.managed servers=[...] -> a correct add/remove diff against the
  device's real NTP servers.

What issues does this PR fix or reference?

Found by an audit of the NAPALM state modules; part of the same family as
#62170. (netsnmp's end-to-end apply additionally needs napalm's py2 SNMP
templates fixed upstream -- napalm-automation/napalm#2326 -- but the state logic
fixed here is independent and was the crashing/silent layer.)

Merge requirements satisfied?

• Docs
• Changelog
• Tests written/updated

New tests/pytests/unit/states/test_netsnmp.py and test_netntp.py (neither
module had any tests): the None-defaults crash, community-mode normalization,
the valid->valid "updated" diff, _check's in-place resolution, and the
masked-retrieval-failure -- each fails against the current code, plus the
behaviour-preserving inverse cases.

Commits signed with GPG?

No