load_payloads: tolerate globbed-but-uncached payload files

[MacrayBlackhand]

Problem

FlyingType.load_payloads() aborts the entire mission load with a KeyError
when a payload directory contains a *.lua file that scan_payload_dir()
globbed but did not cache.

The two halves of the payload machinery use mismatched guards:

• scan_payload_dir() globs every *.lua in each payload dir, but only inserts
  a cache entry when the file content matches the regex
  \["unitType"]\s*=\s*"([^"]*) (dcs/unittype.py, ~L108-122). A file whose
  unitType is written in a form the regex doesn't match is globbed but never
  cached.
• load_payloads() then re-globs the same *.lua files and reads the cache with
  a bare subscript (dcs/unittype.py:140):

  if FlyingType._payload_cache[payload_path] == cls.id and payload_path.exists():

For any globbed-but-uncached path, _payload_cache[path] raises KeyError,
which propagates out of load_payloads() and aborts loading the whole mission.

Concrete current motivating case: the shipping ED F-100D module ships
CoreMods/aircraft/F-100D/UnitPayloads/F-100D.lua. That directory is on the
payload search path, so the file globs -- but its content isn't matched by the
["unitType"] regex, so it isn't cached. The result: loading any mission that
references an F-100D crashes in load_payloads().

Relationship to #317

Issue #317 ("replace our lua parser with lupa?") diagnoses the same underlying
cause -- the regex-based payload scanner can't handle certain payload-file
syntaxes (the F-100D among them) -- and proposes a larger lupa-based parser
rewrite.

This PR is the small, complementary, defensive fix: it does not attempt to fix
the parser or make the uncacheable file parse. It only stops one
unparseable-but-globbed file from taking down the entire mission load, regardless
of whether or when the larger parser rewrite in #317 lands. A file the scanner
can't cache is simply skipped (which is already the intended outcome for a file
that doesn't match a unit) instead of raising.

Fix

One call, in load_payloads():

-                if FlyingType._payload_cache[payload_path] == cls.id and payload_path.exists():
+                if FlyingType._payload_cache.get(payload_path) == cls.id and payload_path.exists():

An uncached path -> .get() returns None -> None != cls.id -> the file is
skipped. Cached files are unaffected; nothing else changes.

Test

Added test_load_payloads_skips_globbed_but_uncached_file to
tests/test_unittype.py (matching the existing tmp_path/monkeypatch style).
It creates a payload directory containing:

• an uncacheable .lua (no ["unitType"]="..." line -- the F-100D failure
  shape), and
• a normal cacheable payload .lua for the unit under test,

points the scanner at the temp directory, and asserts that load_payloads() does
not raise and still loads the valid payload. The test fails with KeyError
against the pre-fix bare subscript and passes with .get().

[rp-]

#439

was faster, sorry