feat(agent): buffer retry/fallback status, surface only on terminal failure (#33816)

Users report that the CLI/gateway floods them with confusing retry chatter
during transient failures: a single 429 can produce 10+ "Provider/Endpoint/
Retrying in 5s..." lines before the request eventually succeeds. The same
firehose hits Telegram, Discord, Slack, etc. via _emit_status.

This patch defers all retry/fallback/compression status messages until we
know the outcome:
  - if the turn ultimately succeeds (any path: primary recovers, fallback
    activates, compression unsticks the request), the buffer is silently
    dropped — the user sees nothing.
  - if every retry and fallback exhausts and the turn fails, the buffer
    is flushed at the terminal-failure return so the user sees the full
    retry trace alongside the final error.

Backend logging (agent.log) is unchanged — every emission site still
writes to logger.warning/info, so post-mortem diagnosis is intact.

## What changed

run_agent.py: four new methods on AIAgent:
  _buffer_status(msg)   — defer an _emit_status call
  _buffer_vprint(msg)   — defer a _vprint(force=True) line
  _clear_status_buffer() — drop pending messages on success
  _flush_status_buffer() — replay pending messages on terminal failure

agent/conversation_loop.py:
  - converted ~30 mid-process emit/vprint sites in the retry, fallback,
    compression, empty-response, and stream-watchdog paths to the buffered
    helpers
  - added _flush_status_buffer() at every terminal-failure return so users
    still see the trace when it actually matters
  - added _clear_status_buffer() at the "non-empty assistant content"
    point (NOT at "API call returned bytes" — empty responses still loop
    through the empty-retry path and would otherwise lose their trace
    between iterations)
  - silenced the two "(´;ω;`) oops, retrying..." / "(╥_╥) error,
    retrying..." spinner final-frame messages — the spinner now stops
    cleanly so retries leave no visible residue

agent/chat_completion_helpers.py: same conversion for codex TTFB / stale-
stream / fallback-activation status messages.

agent/stream_diag.py: _emit_stream_drop now buffers instead of emitting
directly.

## Tests

tests/run_agent/test_retry_status_buffer.py: 7 unit tests covering
accumulate→flush, clear-on-success, mixed kinds, empty-buffer no-op,
re-buffer after flush, exception swallowing.

Updated 3 existing tests that mocked _emit_status to also mock (or use)
_buffer_status:
  - tests/run_agent/test_run_agent.py::test_empty_response_emits_status_for_gateway
  - tests/run_agent/test_stream_drop_logging.py (2 tests)
  - tests/agent/test_codex_ttfb_watchdog.py (TTFB hint test)

## Validation

Live test: hermes chat -q against an unreachable endpoint with no fallback
exhausts retries and prints the full trace at the end. Same flow against
a working endpoint prints zero retry chatter.

agent/chat_completion_helpers.py

@@ -403,13 +403,13 @@ def interruptible_api_call(agent, api_kwargs: dict):
                 _elapsed, _ttfb_timeout, api_kwargs.get("model", "unknown"),
             )
             if _silent_hint:
-                agent._emit_status(
+                agent._buffer_status(
                     f"⚠️ No first byte from provider in {int(_elapsed)}s "
                     f"(codex stream, model: {api_kwargs.get('model', 'unknown')}). "
                     f"Reconnecting. {_silent_hint}"
                 )
             else:
-                agent._emit_status(
+                agent._buffer_status(
                     f"⚠️ No first byte from provider in {int(_elapsed)}s "
                     f"(codex stream, model: {api_kwargs.get('model', 'unknown')}). "
                     f"Reconnecting."
@@ -455,7 +455,7 @@ def interruptible_api_call(agent, api_kwargs: dict):
                 api_kwargs.get("model", "unknown"),
                 f"{_est_tokens_for_codex_watchdog:,}",
             )
-            agent._emit_status(
+            agent._buffer_status(
                 f"⚠️ Codex stream sent no events for {int(_event_stale_elapsed)}s "
                 f"after first byte (model: {api_kwargs.get('model', 'unknown')}). "
                 f"Reconnecting."
@@ -493,13 +493,13 @@ def interruptible_api_call(agent, api_kwargs: dict):
                 api_kwargs.get("model", "unknown"), f"{_est_ctx:,}",
             )
             if _silent_hint:
-                agent._emit_status(
+                agent._buffer_status(
                     f"⚠️ No response from provider for {int(_elapsed)}s "
                     f"(non-streaming, model: {api_kwargs.get('model', 'unknown')}). "
                     f"{_silent_hint}"
                 )
             else:
-                agent._emit_status(
+                agent._buffer_status(
                     f"⚠️ No response from provider for {int(_elapsed)}s "
                     f"(non-streaming, model: {api_kwargs.get('model', 'unknown')}). "
                     f"Aborting call."
@@ -1262,7 +1262,7 @@ def try_activate_fallback(agent, reason: "FailoverReason | None" = None) -> bool
                 api_mode=agent.api_mode,
             )
 
-        agent._emit_status(
+        agent._buffer_status(
             f"🔄 Primary model failed — switching to fallback: "
             f"{fb_model} via {fb_provider}"
         )
@@ -2251,7 +2251,7 @@ def interruptible_streaming_api_call(agent, api_kwargs: dict, *, on_first_delta=
                             mid_tool_call=False,
                             diag=request_client_holder.get("diag"),
                         )
-                        agent._emit_status(
+                        agent._buffer_status(
                             "❌ Provider returned malformed streaming data after "
                             f"{_max_stream_retries + 1} attempts. "
                             "The provider may be experiencing issues — "
@@ -2358,7 +2358,7 @@ def interruptible_streaming_api_call(agent, api_kwargs: dict, *, on_first_delta=
                 _stale_elapsed, _stream_stale_timeout,
                 api_kwargs.get("model", "unknown"), f"{_est_ctx:,}",
             )
-            agent._emit_status(
+            agent._buffer_status(
                 f"⚠️ No response from provider for {int(_stale_elapsed)}s "
                 f"(model: {api_kwargs.get('model', 'unknown')}, "
                 f"context: ~{_est_ctx:,} tokens). "