MINOR: filters: Reject mixing legacy and filter-* directives

MINOR: filters: Reject mixing legacy and filter-* directives

Do not allow filter-config, filter-enable/filter-disable or filter-sequence
together with an explicit legacy filter directive in the same proxy,
regardless of parsing order. A legacy filter must also be rejected when the
proxy inherits explicit filter-* directives from its defaults section.

Mark configurations created by the legacy filter parser so implicit legacy
filters do not select a mode. Share the explicit-instance/directive check
with proxy_ref_defaults(), ignoring FLT_INST_F_IMPLICIT instances on both
sides. Check for conflicts before invoking parsers or recording directives.

Add native VTC coverage for all four directives, both declaration orders,
inherited directives and response-only instances. Also check that each mode
permits multiple declarations, different proxies can use different modes,
and implicit cache filters can coexist with either explicit syntax.