v1.6.2: p Restore gonja v1 filters dropped by the v2 migration

The gonja v2 rewrite removed non-Jinja builtin filters (fromjson, split,
unset) that deployment templates rely on. Port all three to the gonja v2
FilterFunction API and register them into builtins.Filters.

Also set KeepTrailingNewline=true so rendered config files retain their
trailing newline (gonja v1 kept it; v2 strips it by Jinja default).

Add comprehensive unit tests (100% coverage on filters and renderTemplate)
and a golden-file regression suite with synthetic templates exercising every
Jinja construct downstream deployments use.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>