fix(providers): sanitize UTF-16 surrogates at provider request boundary

Symptom
-------
LLM requests intermittently fail with:
  'utf-8' codec can't encode characters in position N-N+1: surrogates not allowed
when messages contain emoji-heavy content (e.g. HTML with mixed emoji + JSON round-trips).
This blocks the affected session until the session file is quarantined.

Root cause
----------
Surrogate sanitization was only applied at the CLI entry point
(nanobot/cli/commands.py: _sanitize_surrogates). Requests entering
the LLM provider layer through other channels (Feishu, cron, webui,
tool results, memory injection) had no defensive cleaning, so any
message that happened to carry unpaired UTF-16 surrogates (from an
upstream JSON round-trip with ensure_ascii=True on ill-formed input,
memory rehydration, or third-party content) would blow up at
json.dumps -> HTTP encode time inside the provider client.

Fix
---
1. Extract sanitize_surrogates() and sanitize_surrogates_deep() into
   nanobot/utils/helpers.py as the single source of truth. Both use
   utf-16-le round-tripping with errors='surrogatepass' / 'replace',
   so paired surrogates reconstruct back into their real code point
   and lone surrogates collapse to U+FFFD.
2. Make nanobot/cli/commands.py:_sanitize_surrogates a thin wrapper
   that re-exports the shared helper (backward compatible).
3. Add defense-in-depth at the LLM provider boundary in
   nanobot/providers/base.py:_sanitize_empty_content by running
   sanitize_surrogates_deep over each message and its content blocks
   right before requests are serialized to JSON.

Non-goals
---------
- truncate_text() is intentionally left untouched. Python str slicing
  cannot split a single code point into surrogate halves, so it is
  not the source of lone surrogates.
- session/manager storage layer is untouched. Archived sessions
  reproduced the failure only through the request path, not through
  storage.

Verification
------------
- New regression suite tests/providers/test_sanitize_surrogates.py
  covers: paired surrogate reconstruction, lone surrogate replacement,
  identity return on clean input (zero allocation), deep recursion on
  dict/list/tuple, provider _sanitize_empty_content integration, and
  full utf-8 encodability of the sanitized request body.
- 14/14 new tests pass; full existing test module also green.
- Replayed 58 archived real session messages plus adversarial
  lone-surrogate injection through the provider path with no encode
  errors after the fix.

Impact
------
- No behaviour change for clean inputs (sanitize_surrogates_deep is
  an identity return when no surrogate is present).
- Fails-safe: unpaired surrogates degrade to U+FFFD instead of
  aborting the entire request.

nanobot/cli/commands.py

@@ -78,7 +78,12 @@ from nanobot.config.paths import get_workspace_path, is_default_workspace  # noq
 from nanobot.config.schema import Config  # noqa: E402
 from nanobot.security.network import is_loopback_host  # noqa: E402
 from nanobot.utils.evaluator import evaluate_response, resolve_evaluator_prompt  # noqa: E402
-from nanobot.utils.helpers import sync_workspace_templates  # noqa: E402
+from nanobot.utils.helpers import (  # noqa: E402
+    sanitize_surrogates as _sanitize_surrogates,
+)
+from nanobot.utils.helpers import (  # noqa: E402
+    sync_workspace_templates,
+)
 from nanobot.utils.restart import (  # noqa: E402
     consume_restart_notice_from_env,
     format_restart_completed_message,
@@ -92,17 +97,6 @@ from nanobot.webui.build import (  # noqa: E402
 from nanobot.webui.sidebar_state import read_webui_sidebar_state  # noqa: E402
 
 
-def _sanitize_surrogates(text: str) -> str:
-    """Reconstruct surrogate pairs into real characters; replace lone surrogates.
-
-    On Windows, console input may produce lone surrogate code points (e.g.
-    ``\\ud83d\\udc08`` for U+1F408).  Round-tripping through UTF-16 reconstructs
-    paired surrogates into their actual characters and replaces unpaired ones
-    with U+FFFD.
-    """
-    return text.encode("utf-16-le", errors="surrogatepass").decode("utf-16-le", errors="replace")
-
-
 def _signal_name(signum: int) -> str:
     with suppress(ValueError):
         return signal.Signals(signum).name