fix(agent): keep MoA verdicts fallback-free; nest the gated fallback under one if

Two CI regressions from the previous commit, both mine:

- `_moa_special_cases` was switched to `_ABORT_FALLBACK`, which flips
  `should_fallback` to True for the MoA adapter-shape and missing-preset
  verdicts. #55933 made those deliberately NOT fall back (a fallback would
  silently replace the MoA route with a single model); restore
  `retryable=False` only. The gate in `settle_unrecovered_error` now honours
  that for real: on main these verdicts never reached the fallback branch
  because the flag was never consulted.
- `tests/agent/test_prompt_cache_ttl_propagation.py` pins every
  `_try_activate_fallback` reference to a direct `if agent._try_activate_fallback():`
  site (#84733 restart discipline); `fallback_allowed and agent._try_activate_fallback()`
  broke that shape. Nest the two calls under one `if should_fallback or local_validation:`
  block instead.

agent/error_classifier.py

@@ -609,10 +609,10 @@ def _moa_special_cases(c: _Ctx) -> Optional[Verdict]:
     # Local MoA streaming adapter-shape bugs are not a provider outage; falling
     # back would silently replace the MoA route with a single model (#55933).
     if c.provider_slug == "moa" and any(s in str(c.error) for s in _MOA_ADAPTER_SHAPE_BUGS):
-        return _v(_R.format_error, **_ABORT_FALLBACK)
+        return _v(_R.format_error, retryable=False)
     # Persisted MoA preset name that was renamed/deleted — deterministic config error.
     from agent.errors import MoAPresetNotFoundError
-    return _v(_R.model_not_found, **_ABORT_FALLBACK) if isinstance(c.error, MoAPresetNotFoundError) else None
+    return _v(_R.model_not_found, retryable=False) if isinstance(c.error, MoAPresetNotFoundError) else None
 
 
 def _by_error_code(c: _Ctx) -> Optional[Verdict]: