feat(memory): add memory.token_counting config to avoid tiktoken network dependency (#3429) (#3465)

* feat(memory): add memory.token_counting config to avoid tiktoken network dependency (#3429)

Add a `memory.token_counting` option (`tiktoken` | `char`) so deployments in
network-restricted environments can opt out of tiktoken entirely. In `char`
mode the memory-injection budget uses a network-free character-based estimate
and never triggers the BPE download from openaipublic.blob.core.windows.net,
which could otherwise block for tens of minutes (see #3402).

Also harden the default `tiktoken` path:
- cache an in-flight LOADING sentinel so concurrent callers fall back
  immediately instead of spawning more blocking get_encoding threads when the
  first load is still running (e.g. under the 5s startup warm-up timeout);
- cache failures with a timestamp and retry after a cooldown so a transient
  network outage self-heals back to accurate counting without a restart;
- skip startup warm-up entirely in char mode.

The new config is surfaced via the memory config API and config.example.yaml
(config_version bumped). Default remains `tiktoken`, so existing deployments
are unaffected.

* fix(memory): use CJK-aware char token estimate and address review feedback

- Replace the flat len(text)//4 fallback with a CJK-aware estimate so
  Chinese/Japanese/Korean memory content does not over-fill the injection budget
- Document the internal tiktoken retry cooldown and char-mode escape hatch
- Sync CLAUDE.md / config.example.yaml / MEMORY_IMPROVEMENTS.md wording
- Fix MemoryConfigResponse mocks/assertions and add CJK estimate tests

backend/app/gateway/routers/memory.py

@@ -98,6 +98,7 @@ class MemoryConfigResponse(BaseModel):
     fact_confidence_threshold: float = Field(..., description="Minimum confidence threshold for facts")
     injection_enabled: bool = Field(..., description="Whether memory injection is enabled")
     max_injection_tokens: int = Field(..., description="Maximum tokens for memory injection")
+    token_counting: str = Field(..., description="Token counting strategy for memory injection ('tiktoken' or 'char')")
 
 
 class MemoryStatusResponse(BaseModel):
@@ -310,7 +311,8 @@ async def get_memory_config_endpoint() -> MemoryConfigResponse:
             "max_facts": 100,
             "fact_confidence_threshold": 0.7,
             "injection_enabled": true,
-            "max_injection_tokens": 2000
+            "max_injection_tokens": 2000,
+            "token_counting": "tiktoken"
         }
         ```
     """
@@ -323,6 +325,7 @@ async def get_memory_config_endpoint() -> MemoryConfigResponse:
         fact_confidence_threshold=config.fact_confidence_threshold,
         injection_enabled=config.injection_enabled,
         max_injection_tokens=config.max_injection_tokens,
+        token_counting=config.token_counting,
     )
 
 
@@ -351,6 +354,7 @@ async def get_memory_status() -> MemoryStatusResponse:
             fact_confidence_threshold=config.fact_confidence_threshold,
             injection_enabled=config.injection_enabled,
             max_injection_tokens=config.max_injection_tokens,
+            token_counting=config.token_counting,
         ),
         data=MemoryResponse(**memory_data),
     )