rpadmin: add PreRestartProbe and PostRestartProbe

[david-yu]

Summary

Adds Go bindings for the broker-local restart probes introduced in
Redpanda 25.1
(src/v/redpanda/admin/api-doc/broker.json):

Endpoint	New method	Response
GET /v1/broker/pre_restart_probe	PreRestartProbe(ctx, limit)	PreRestartCheckResult{Risks}
GET /v1/broker/post_restart_probe	PostRestartProbe(ctx, limit)	PostRestartCheckResult{LoadReclaimedPercent}

PreRestartProbe asks the broker whether restarting it right now would
affect partitions, and groups the affected partitions into four risk
categories:

• rf1_offline — RF=1 partitions go offline (generally acceptable risk)
• full_acks_produce_unavailable — acks=-1 produce would be rejected
• unavailable — both produce and consume rejected
• acks1_data_loss — acks=1 produce could lose data

PostRestartProbe reports how much load this broker has reclaimed
since the most recent restart as a percentage of in-sync replicas.

Both probes answer for the broker that handles the request, so callers
typically scope the client to a specific broker via ForHost before
invoking. The optional limit parameter caps the number of partitions
returned per risk category (server default is 128).

Why

The K8s operator's rolling-restart gate is the immediate consumer
(redpanda-operator#1537):
today it relies on the cluster-wide health overview, which lags pod
state and conflates "this broker isn't safe to restart" with "the
cluster has any under-replicated partition." PreRestartProbe gives
the operator a precise per-decision signal; PostRestartProbe lets it
defer the next pod delete until the just-restarted broker has actually
caught up rather than just passing a liveness check.

Tracked in Redpanda's ENG-222.

Testing

Per review feedback, the probes are now exercised end-to-end against a
real Redpanda broker rather than an httptest mock. The integration
tests in rpadmin/api_broker_test.go spin up
redpandadata/redpanda-nightly:latest via testcontainers-go's redpanda
module
(same dependency already used by kvstore and redpanda-otel-exporter)
and drive the rpadmin client against the container's admin API.

• TestPreRestartProbe
  • creates a 4-partition RF=1 topic via kadm
  • asserts each partition surfaces in rf1_offline (polled, since the
    probe trails topic creation briefly)
  • asserts the other three risk-category fields decode (catching silent
    JSON-tag drift between client and server)
  • asserts limit=1 caps every category to ≤1 entry
• TestPostRestartProbe
  • polls the probe until it succeeds against a freshly-started broker
  • asserts load_reclaimed_pc ∈ [0, 100]
  • asserts limit propagates without the broker rejecting the request

Both tests skip under go test -short and gate behind testcontainers'
Docker discovery so contributors without Docker can still run the rest
of the rpadmin suite. The probe doc comments now also link directly to
the admin API spec (broker.json) for callers who want to verify the
response shape without spelunking the rpadmin source.

CI: the Test workflow already runs the rpadmin matrix entry on every
PR touching **.go; this PR also broadens its trigger paths to include
**/go.mod and **/go.sum so dependency-only updates run the tests too.

Test plan

• go test -run "TestPreRestartProbe|TestPostRestartProbe" ./...
  against redpandadata/redpanda-nightly:latest — both green
• go test -short ./... — integration tests skip cleanly
• go vet ./...
• golangci-lint run --new-from-rev=main ./... — 0 issues

🤖 Generated with Claude Code

[secpanda]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues

💻 Catch issues earlier using the plugins for VS Code, JetBrains IDEs, Visual Studio, and Eclipse.

[andrewstucki]

Can you either swap this to run a real broker via something like testcontainers or at least link to the restart probe API docs?

[david-yu]

Can you either swap this to run a real broker via something like testcontainers or at least link to the restart probe API docs?

Will do, will run a real broker with testcontainers. Thank you.