fix(server): default a zero SSE write timeout to 120s - #1686

Linked issue

• Closes: adkrest/agentengine: zero SSEWriteTimeout makes every response fail #1685

2. Describe the change:

Problem:

RuntimeAPIController and AgentEngineAPIController set the write deadline to time.Now().Add(sseTimeout). When the timeout is left at zero (the zero value of adkrest.ServerConfig.SSEWriteTimeout, or 0 passed to agentengine.NewHandler), the deadline has already passed, so every /run_sse response and every Agent Engine query is lost and the client sees EOF. The launcher always passes 120s, so it only hits people who build the server themselves, and the field was undocumented.

Solution:

Treat a zero or negative timeout as 120s, the launcher's --sse-write-timeout default, in both controller constructors (same place an unset Authorizer gets its default). Document the behaviour on ServerConfig.SSEWriteTimeout and agentengine.NewHandler.

I went with a default rather than "zero means no deadline" (as in http.Server.WriteTimeout) to keep the slow-client protection. Happy to switch if maintainers prefer the other.

Behavior change

A server built without a timeout now serves /run_sse and Agent Engine queries with a 120s write deadline instead of failing every request. Servers that set a positive timeout are unchanged.

Testing Plan

Unit Tests:

• All unit tests pass locally.

With your source change reverted and your tests kept, which test fails?

--- FAIL: TestRunSSEWithoutSSEWriteTimeout (0.00s)
    sse_timeout_integration_test.go:51: POST /run_sse error = Post "http://127.0.0.1:61856/run_sse": EOF
--- FAIL: TestQueryWithoutSSEWriteTimeout (0.00s)
    handler_test.go:93: POST /reasoning_engine error = Post "http://127.0.0.1:61915/reasoning_engine": EOF

Both run against a real httptest.Server, since httptest.ResponseRecorder does not support write deadlines.

With the change, go test -race ./server/... ./cmd/... passes, and gofmt / golangci-lint are clean.

Manual End-to-End (E2E) Tests:

Found while embedding adkrest.NewServer in a small program without setting the timeout: /run_sse returned EOF until SSEWriteTimeout was set.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.