fix(replay-e2e): match by conversation, not the living system prompt (#3436)

* fix(replay-e2e): match by conversation, not the living system prompt

The model-replay match key hashed the full input including the lead-agent
system prompt. That prompt is edited frequently (e.g. #3195 added a "File
Editing Workflow" section), so the committed fixture went stale the moment
the prompt changed on main — turning the Layer-2 render gate RED on every
unrelated PR (#3430, #3432, ...). This was a self-inflicted false positive.

Root-cause fix:
- replay_provider._canonical_messages now EXCLUDES the system message from
  the hash. The conversation (human/ai/tool) is the stable contract that
  identifies a recorded turn; the system prompt is an internal detail not
  part of the front-back contract under test. (Mirrors how open-design keys
  its mock picker on the user prompt, not the system internals.) Proven
  robust: injecting a prompt edit no longer causes a replay miss.
- Layer-1 golden was BLIND to replay misses: the gateway swallows a miss
  into an assistant error message, so the shape-only golden stayed green on
  a stale fixture. It now inspects replay_provider.replay_misses() and fails
  loud. (Layer-2 already fails on a miss.)
- Re-recorded write_read_file.ultra fixture + regenerated golden under the
  new conversation-only hash.
- Layer-2 render spec: assert the in-graph auto-title (deterministic); the
  follow-up suggestion is fired async and depends on a clean JSON model
  output, so assert it only when the fixture captured one — never gate on
  its absence (recording flakiness must not block CI).
- docs: REPLAY_E2E.md updated.

Verified: Layer-1 golden green (no miss), Layer-2 both specs green,
CI=true make test 4033 passed / 0 failed, frontend pnpm check clean.

* test(replay-e2e): restore suggestions coverage with a reliable capture

Addresses review feedback (the suggestion path was dropped from Layer-2):

- record spec now waits for the `/suggestions` response before checking
  capture stability, so the recorded fixture reliably includes the
  frontend-fired suggestions turn (previously the stability window could
  return before suggestions fired, yielding a fixture without it).
- Re-recorded write_read_file.ultra: 5 turns (write_file, auto-title,
  read_file, answer, suggestions). Golden unchanged — suggestions is a
  separate /suggestions call, not part of the /runs/stream SSE sequence.
- Layer-2 spec: restore the hard `EXPECTED_SUGGESTION` assertion. With the
  record spec now waiting for /suggestions, a fixture missing the suggestion
  turn means a broken recording and must fail loud, not pass silently.

Verified: Layer-1 golden green (no miss), Layer-2 both specs green
(auto-title + suggestion render), frontend pnpm check clean.

* ci: re-trigger (flaky Docker Hub image pull in sandbox e2e, unrelated)

backend-unit-tests failed only in test_sandbox_orphan_reconciliation_e2e.py
with 'docker pull busybox:latest ... context deadline exceeded' — a CI-runner
network flake reaching Docker Hub, not related to this docs/tests-only change.
Empty commit to re-run CI.

---------

Co-authored-by: DanielWalnut <45447813+hetaoBackend@users.noreply.github.com>

backend/docs/REPLAY_E2E.md

@@ -50,12 +50,25 @@ gateway's own run/event stores using the request's auth context, so the real
 ## How replay works
 
 `tests/replay_provider.py::ReplayChatModel` returns recorded assistant turns keyed
-by a **normalized hash** of the model input (strips `<system-reminder>`, dates,
-UUIDs, tmp paths). A miss raises loudly rather than passing silently. The system
-prompt is made environment-independent by pinning skills + extensions empty and
-disabling memory/summarization (`tests/_replay_fixture.py::build_config_yaml`), so
-a fixture replays the same across machines, days, and CI. Replaying needs **no
-API key**.
+by a **normalized hash of the conversation** (human / ai / tool messages — role,
+text, tool-call name+args; with `<system-reminder>`, dates, UUIDs, tmp paths
+stripped). A miss raises loudly rather than passing silently.
+
+**The system prompt is excluded from the match key.** The lead-agent system
+prompt is a living, frequently-edited implementation detail — its wording changes
+across PRs (e.g. #3195 added a "File Editing Workflow" section). Hashing it would
+make every fixture go stale and red-fail unrelated PRs the moment anyone edits the
+prompt. The conversation flow (user input → tool calls → results → answer) is the
+stable contract that identifies a recorded turn. (This mirrors how open-design's
+mock picker keys on the user prompt, not the system internals.) Combined with
+pinning skills + extensions empty and disabling memory/summarization
+(`tests/_replay_fixture.py::build_config_yaml`), a fixture replays the same across
+machines, days, prompt edits, and CI. Replaying needs **no API key**.
+
+A swallowed hash-miss keeps the SSE *event shapes* identical (the gateway wraps it
+into a normal assistant error message), so the Layer-1 golden can't catch a miss
+by shape alone — it inspects `replay_provider.replay_misses()` and fails loud
+instead. Layer-2 already fails on a miss (the recorded turns never render).
 
 ## Record a new scenario (needs a real key — dev machine only)