fix(HybridBridge): unwrap a Header's JsonContent/XmlContent in hybrid mode

[DerManoMann]

Overview

Hybrid mode silently dropped a Header's content when that content was written as a
JsonContent or XmlContent. The header itself came out, description and all, simply
without its content — and with no warning or error to say so. Classic and spec both
render it.

Header gained content in #2191, which also taught the merge processors to accept it.
Hybrid runs with an empty processor pipeline, so MergeJsonContent never fires: the
annotation stays parked in _unmerged while $header->content is still the undefined
sentinel. HybridBridge::convertHeader() read that property directly, found the sentinel,
and emitted the header with nothing in it.

The second half of this is documentation. Writing the fixture raised a question the suite
had never answered in writing — when a new case earns a new Scratch fixture rather than a
case in an existing one — so that rule is now written down, together with a marker for the
test scaffolding that exists only to serve classic and hybrid.

Changes

• HybridBridge::resolveContent() takes OA\Header in its union, and convertHeader()
  calls it rather than mapping over the raw property. The unwrapping it already does for
  Response, RequestBody and Parameter is exactly what Header was missing.
• Clears the two argument.type errors on convertHeader() that chore(Types): clear phpstan level 7 outside classic #2203 left standing
  deliberately; level 7 on src/HybridBridge.php goes from 17 errors to 15.
• Scratch/HeaderObject carries two more headers, one reaching content through
  JsonContent and one through XmlContent. It pinned content in all three modes
  already, but only via an explicit MediaType list — the one shape that never reaches the
  merge processors, which is why this went unseen.
• docs/dev/testing.md gains a rule for what belongs in a Scratch fixture: the unit is the
  field family with the attribute as the default, a new case belongs in the existing fixture
  unless it needs expected logs the rest of the file must not inherit or cannot be written in
  the spec attribute API, and a fixture is named for the shape it holds rather than the bug
  that prompted it. Plus the trap that Scratch fixtures are deliberately not autoloadable,
  which makes bin/openapi report one as an unknown class instead of running it.
• ROADMAP.md defines a [classic] / [hybrid] marker for documentation and test structure
  that @deprecated cannot reach, so the v8 removal is a grep rather than an untangling. Its
  first use is the new testing.md section.