Test the deadline is refreshed per-frame, not just extended (#87)

Gadfly re-review: the regression test proved the stream outlives the server
WriteTimeout, but its whole run was under a second — far below the 30s
sseWriteTimeout — so it could NOT tell a per-frame refresh from a deadline set
once at open. A revert to set-once would reintroduce the unbounded-block risk
the per-frame refresh exists to prevent, and sail through the test.

Make sseWriteTimeout a var (production never reassigns it) so a test can shrink
it, and split into two focused cases sharing a streamFrames helper:

- TestEventStreamOutlivesServerWriteTimeout — server WriteTimeout 300ms, frames
  straddling it, sseWriteTimeout left at 30s. Catches #78 (no deadline
  management → server cuts the stream). Huge margin, so CI slowness only makes
  it pass more surely — addresses the "timing margin tight" finding too.
- TestEventStreamRefreshesDeadlinePerFrame — sseWriteTimeout shrunk to 400ms,
  8 frames at a 100ms tick (800ms total, 4× margin per gap). A per-frame refresh
  delivers all 8; a set-once deadline expires mid-stream and cuts it.

Verified each fails against its own regression: removing the per-write refresh
gives 3/8 on the per-frame test (EOF at ~400ms); removing both deadline calls
gives 0/3 on the outlives test. Both pass on the real code, 3× under -count.

The "clear the deadline entirely" finding was already addressed by the previous
commit (per-frame refresh); this covers it against reintroduction.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

internal/api/agent.go

@@ -117,7 +117,11 @@ func (h *handlers) agentChat(c *gin.Context) {
 // stream's mutex with it, and taking the keep-alive down too since it needs the
 // same lock. Generous, because it is a backstop against a stuck peer and not a
 // pacing mechanism.
-const sseWriteTimeout = 30 * time.Second
+//
+// A var, not a const, ONLY so the test can shrink it to prove the deadline is
+// refreshed per frame rather than set once — a set-once 30s deadline would pass
+// a test whose whole run is under a second. Production never reassigns it.
+var sseWriteTimeout = 30 * time.Second
 
 // eventStream serializes writes to one SSE response.
 //