ui: FPLMN manual-selection clear and duplicate guard (v2.7.19)

Attaching to a PLMN listed in EF.FPLMN used to write successful locations
anyway, i.e. it attached to a forbidden network.  Per TS 23.122 a
successful manual selection removes the entry, so the attach scenarios
clear it first:

- netsim.remove_fplmn() clears every occurrence of the PLMN (entries are
  not compacted; FFFFFF gaps stay); insert_fplmn() returns None when the
  PLMN is already listed, so roaming_denied no longer stores duplicates
  (the live card had '250-99, 250-99').
- clear_fplmn() step, called from write_real_locations() -> covers
  attach_eps, attach_2g and the sms_received location rewrite; the write
  is logged as a normal fplmn update_binary.
- tests: remove_fplmn duplicates/absent/gaps; roaming_denied duplicate
  skip; attach clears both occurrences before the location writes;
  make_runner now copies FakeFileInfo so seeded data does not leak
  between tests.
- help EN/RU and AGENTS updated.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "pysim-simple-server"
-version = "2.7.18"
+version = "2.7.19"
 description = "HTTP REST server wrapping pysim for the SIMple PWA"
 requires-python = ">=3.8"
 # pysim is a git-only dependency installed explicitly by setup.bat/setup.sh.