test: prove SSE first-delta delivery at the API boundary

[Chris0Jeky]

Add controller-level tests that prove the first SSE delta is readable before the provider completes. A gated provider and ResponseHeadersRead make the assertion deterministic; the tests also check event order, JSON payloads, blank-line framing, a single text/event-stream content type and compatibility with an immediately available response.

This is the AC3 API-test slice of #2241. It does not change production chat code or claim a real proxy/socket buffering test, a live provider smoke, or the whole issue's acceptance.

Validation at d0ba6c9: 2 focused tests passed; full dotnet test backend/Taskdeck.sln -c Release -m:1 exited 0 with 8,975 passed, 5 existing skips, 0 failures. Initial proving exposed a missing System.Net.Http.Json import and a test payload-casing assumption; the two test-only corrections are committed before publication. Final project counts: Domain 1605, Application 4206, API 2857 plus 4 skips, CLI 243, Architecture 28 plus 1 skip, Integration 36.

The original deterministic handshake and wire-framing implementation received independent Terra review without a C/H blocker. A scoped Luna check covers the subsequent test-only import/deserialization corrections. Hosted exact-head CI remains required before merge. Logs and exit receipts will be preserved outside the disposable worktree; no private data or provider credentials were used.

Refs #2241

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-08T01:53:15.152221Z	d0ba6c9	PR opened

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[Chris0Jeky]

Final scoped Luna verification completed at d0ba6c9 for the corrections after the original Terra-reviewed c80a064 head. Two focused tests passed; typed LlmTokenEvent parsing matches the controller serialization, and the gated handshake, frame order and payload assertions remain meaningful. No causal C/H correctness, false-positive or hang blocker. Optional TokensUsed/Provider/Model fields are not individually asserted: informational coverage scope, not a defect or new fix request. Author full backend proof remains 8,975 passed / 5 existing skips at this final head. Required hosted CI is still separate.

[Chris0Jeky]

Session wrap-up: all source is pushed at d0ba6c9. Focused and full test logs/exit receipts plus generated OpenAPI evidence were copied outside the disposable checkout to C:\Users\Public\codex-shell-home\taskdeck-wave-20260908\issue-2241; test-file SHA-256 copies were verified before cleanup. The clean checkout will be removed normally by the coordinator; generated bin/obj caches are disposable. Final fix verification is recorded above. This ready PR remains open for required CI and next-session reconciliation; no monitor or test process remains owned by this session.