Skip saltutil runner/wheel privilege drop on invalid user (#69600)

[dwoz]

What does this PR do?

Make saltutil.runner / saltutil.wheel skip the master-user privilege drop
when the configured user is not a real account on the system, falling back
to the historical in-process behavior.

What issues does this PR fix or reference?

Fixes #69600 (regression from #69240 / #67716)

Previous Behavior

Running an orchestration under sudo -- e.g.

sudo salt-run state.orchestrate orch.os_updates pillar='{"env":"mgmt"}'

-- whose rendered SLS called salt.saltutil.runner('cache.grains', ...)
from Jinja failed with:

Failed to run 'cache.grains' as user 'sudo_alice':
KeyError: "getpwnam(): name not found: 'sudo_alice'"

state.orchestrate overwrites __opts__["user"] with __user__ (the
publishing user, salt.utils.user.get_specific_user(), which returns
"sudo_<login>" when invoked under sudo). The privilege-drop helper
introduced in #69240 read that value as the runas target and asked
chugid to switch to it, raising KeyError from pwd.getpwnam.

New Behavior

_master_user_runas now validates the candidate against the passwd
database and returns None (skip the drop) when it does not resolve to
a real user. Orchestrations launched under sudo therefore keep
working; the privilege drop still triggers when the master is genuinely
configured to run as a different real user (the case the original fix
intended to cover).

Merge requirements satisfied?

• Changelog
• Tests written/updated

Commits signed with GPG?

No