chore: merge upstream/main (baf14b9, unique operationIds)

[nagaame]

Why

Fork sync: merge google/gnostic baf14b9 ("fix: operationId must be unique (google#478)")
into main.

main was 10 commits ahead / 1 behind the merge-base e0e09f7, so this is a real merge
commit — a fast-forward, the web "Sync fork" button and a plain git pull cannot do it.
Upstream's change: the 2nd and later http rules of a method now emit
operationId: <op>_<N> so operationIds stay unique (OpenAPI v3.0.3 4.7.10.1).

Conflict resolution

cmd/protoc-gen-openapi/generator/generator.go (1 hunk): the @sse feature (7922890)
added an isSSE argument to buildOperationV3, while baf14b9 switched the very same
call site to the new opID variable. Both changes are kept:

op, path2 := g.buildOperationV3(
    d, opID, service.GoName, comment, defaultHost, path, body, inputMessage, outputMessage, isSSE)

.../additional_bindings/openapi.yaml auto-merged: the infoURL line and upstream's
Messaging_UpdateMessage_2 line are far apart.

Fixture follow-up (2nd commit)

baf14b9 updates upstream's own additional_bindings/openapi.yaml, but this repo also
checks in the fq_schema_naming / json / string_enum / default_response variants for
every test dir, and plugin_test.go diffs every fixture that exists. Those four still
encoded the duplicate operationId and failed. Regenerated via GENERATE_FIXTURES=true
(4 files, 1 line each) — identical to upstream's own fixture change.

Verification

$ go build -mod=vendor ./...            # exit 0
$ go test  -mod=vendor -count=1 ./...   # ok=17  FAIL=0

The four previously failing subtests (TestOpenAPIFQSchemaNaming, TestOpenAPIJSONNaming,
TestOpenAPIStringEnums, TestOpenAPIDefaultResponse — all Additional_Bindings) pass.

Net diff against main is exactly 6 files and contains nothing beyond upstream's own patch
plus those 4 fixture lines: no fork customization was lost.

⚠️ Merge with a merge commit, not squash

This branch carries a real merge commit (a08ae5a) whose second parent is baf14b9.
Squash-merging would drop baf14b9 from main's ancestry, leaving the merge-base at
e0e09f7 — every future git merge upstream/main would then re-deliver that commit and
re-examine everything since e0e09f7. Merge commit also matches how this repo synced
upstream before (562c644).