Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@internal/responsecache/sse_validation_test.go`:
- Around line 12-64: Add a new table-driven test case to the existing test cases
slice in sse_validation_test.go that verifies empty input is rejected: add an
entry with name "rejects empty input", raw: []byte("") and want: false so the
SSE validation logic (the same test harness used for the other cases) explicitly
asserts that an empty payload returns false.

In `@internal/responsecache/sse_validation.go`:
- Around line 28-40: The loop accepts events after a seen `[DONE]` because the
sawDone check runs after early-continue paths (comments/keepalives and
empty-data), so move the sawDone check so it runs immediately after calling
sseEventPayload(event) (i.e., right after "payload, hasData :=
sseEventPayload(event)") and before any continue; this ensures that once sawDone
is set (when payload equals cacheDonePayload) any subsequent event — including
comment/keepalive-only or another `[DONE]` — triggers an immediate false return.
- Around line 43-46: The current validation unmarshals into a map using
json.Unmarshal(payload, &decoded), allocating an unused map and incorrectly
rejecting valid non-object JSON; replace that logic with a syntax-only check
using json.Valid(payload) and remove the unused decoded variable so the function
simply returns false when json.Valid(payload) is false, otherwise proceeds as
valid. Ensure you reference the existing payload variable and remove the
json.Unmarshal call and decoded declaration, using json.Valid(payload) instead.