fix: load default provider config when summarizer uses different prov… (#9051)

fix: load default provider config when summarizer uses different provider

**Problem:**
Summarization failed when agent used one provider (e.g., Google AI) but
summarizer config specified a different provider (e.g., Anthropic):

```python
# Agent LLM config
model_endpoint_type='google_ai', handle='gemini-something/gemini-2.5-pro',
context_window=100000

# Summarizer config
model='anthropic/claude-haiku-4-5-20251001'

# Bug: Resulting summarizer_llm_config mixed Google + Anthropic settings
model='claude-haiku-4-5-20251001', model_endpoint_type='google_ai',  # ❌ Wrong endpoint!
context_window=100000  # ❌ Google's context window, not Anthropic's default!
```

This sent Claude requests to Google AI endpoints with incorrect parameters.

**Root Cause:**
`_build_summarizer_llm_config()` always copied the agent's LLM config as base,
then patched model/provider fields. But this kept all provider-specific settings
(endpoint, context_window, etc.) from the wrong provider.

**Fix:**
1. Parse provider_name from summarizer handle
2. Check if it matches agent's model_endpoint_type (or provider_name for custom)
3. **If YES** → Use agent config as base, override model/handle (same provider)
4. **If NO** → Load default config via `provider_manager.get_llm_config_from_handle()` (new provider)

**Example Flow:**
```python
# Agent: google_ai/gemini-2.5-pro
# Summarizer: anthropic/claude-haiku

provider_name = "anthropic"  # Parsed from handle
provider_matches = ("anthropic" == "google_ai")  # False ❌

# Different provider → load default Anthropic config
base = await provider_manager.get_llm_config_from_handle(
    handle="anthropic/claude-haiku",
    actor=self.actor
)
# Returns: model_endpoint_type='anthropic', endpoint='https://api.anthropic.com', etc. ✅
```

**Result:**
- Summarizer with different provider gets correct default config
- No more mixing Google endpoints with Anthropic models
- Same-provider summarizers still inherit agent settings efficiently

👾 Generated with [Letta Code](https://letta.com)

Co-authored-by: Letta <noreply@letta.com>

tests/managers/test_agent_manager.py

@@ -406,8 +406,14 @@ async def test_compaction_settings_model_uses_separate_llm_config_for_summarizat
         tool_rules=None,
     )
 
-    # Use the static helper on LettaAgentV3 to derive summarizer llm_config
-    summarizer_llm_config = LettaAgentV3._build_summarizer_llm_config(
+    # Create a mock agent instance to call the instance method
+    mock_agent = Mock(spec=LettaAgentV3)
+    mock_agent.actor = default_user
+    mock_agent.logger = Mock()
+
+    # Use the instance method to derive summarizer llm_config
+    summarizer_llm_config = await LettaAgentV3._build_summarizer_llm_config(
+        mock_agent,
         agent_llm_config=agent_state.llm_config,
         summarizer_config=agent_state.compaction_settings,
     )