[ashtonian]

Unsupported bodyless GET requests currently return empty HTTP 200, record a bogus
event and EOF, and abort current updater jobs at blocked_versions. Return HTTP
501 before event decoding so optional read APIs exercise their existing
unavailable-service fallback. Unknown POST event recording is preserved.

Fixes #692.

Validation: the identical two-case regression fails on 41663d5a with empty 200
and EOF/event output; the fixed internal/server package passes normal and race
tests (-p=1 -parallel=2 -count=1). Existing unknown POST coverage passes.
A matching real updater replay with the single CLI repair progresses past this
API error and reaches the separate, preexisting path-dependency failure for
golang.org/x/sys. The broader grouped job reaches its five-minute deadline;
that incomplete run is not a successful dependency-update claim. No updater
experiment or image trust policy is changed.

CI follow-through: go-graph-replace-subproject job 126694083932 failed while
cloning the smoke repository because its proxy at 172.18.0.2:1080 refused
connection. rust-toolchain-plaintext job 126694089811 failed before the CLI
binary ran: proxy.golang.org returned HTTP/2 INTERNAL_ERROR downloading
golang.org/x/net@v0.59.0. Both belong to run
41864195510; no
failed source assertion is shown in either trace. The contributor has READ
permission, so a maintainer must retry these failed jobs. Existing passing jobs
and source inputs should be retained. No full-suite replay or replacement push
was made.