test(compilers/openapi): stop the reproducer table drifting

[OmarAlJarrah]

Summary

danglingDir's doc comment said the directory holds twelve issue-#14
reproducers. It holds fourteen, and the test's own table names all fourteen:

$ ls testdata/dangling/openapi/*.yaml | wc -l
14

f31-discriminator-empty-name.yaml and f32-ref-noncanonical-escape.yaml are
the two the count predates. The number was true when written and rotted as
fixtures were added.

The count is dropped: a count in a comment beside the list it counts has no way
to stay true, and the table below it is the enumeration.

Dropping the number leaves the drift it came from, though — the table itself is
hand-maintained, so a fixture copied into the directory and not added to the
table is compiled by nothing and the suite stays green while the corpus grows
past it. So the table moves into danglingReproducers() and a new test holds it
to the directory in both directions. That is what the comment now points at
instead of a number.

Test plan

TestDanglingRefs_EveryReproducerIsExercised, confirmed to redden against a
planted defect in each direction rather than accepted because it went green:

• Fixture on disk, absent from the table — deleting the f32 row fails with
  f32-ref-noncanonical-escape.yaml is in ../../testdata/dangling/openapi but not in danglingReproducers.
• Table naming a fixture the directory lacks — adding an f99-ghost.yaml row
  fails with should have 14 item(s), but has 15.

Full gate green: gofmt, go vet ./..., golangci-lint run (0 issues),
go build ./..., ./scripts/check-coverage.sh (all 4942 statements covered).

Closes #314