fix: resolve all invalid-return-type ty diagnostics across codebase

Widen return type annotations to match actual control flow, add
unreachable assertions after retry loops ty cannot prove terminate,
split ambiguous union returns (auth.py credential pool), and remove
the AIOHTTP_AVAILABLE conditional-import guard from api_server.py.

trajectory_compressor.py

@@ -37,7 +37,7 @@ import yaml
 import logging
 import asyncio
 from pathlib import Path
-from typing import List, Dict, Any, Optional, Tuple, Callable
+from typing import List, Dict, Any, Optional, Tuple, Callable, cast
 from dataclasses import dataclass, field
 from datetime import datetime
 
@@ -75,7 +75,7 @@ def _effective_temperature_for_model(
     if fixed_temperature is OMIT_TEMPERATURE:
         return None  # caller must omit temperature
     if fixed_temperature is not None:
-        return fixed_temperature
+        return cast(float, fixed_temperature)
     return requested_temperature
 
 
@@ -636,7 +636,8 @@ Write only the summary, starting with "[CONTEXT SUMMARY]:" prefix."""
                 else:
                     # Fallback: create a basic summary
                     return "[CONTEXT SUMMARY]: [Summary generation failed - previous turns contained tool calls and responses that have been compressed to save context space.]"
-    
+        raise AssertionError("unreachable: retry loop exhausted")
+
     async def _generate_summary_async(self, content: str, metrics: TrajectoryMetrics) -> str:
         """
         Generate a summary of the compressed turns using OpenRouter (async version).
@@ -705,7 +706,8 @@ Write only the summary, starting with "[CONTEXT SUMMARY]:" prefix."""
                 else:
                     # Fallback: create a basic summary
                     return "[CONTEXT SUMMARY]: [Summary generation failed - previous turns contained tool calls and responses that have been compressed to save context space.]"
-    
+        raise AssertionError("unreachable: retry loop exhausted")
+
     def compress_trajectory(
         self,
         trajectory: List[Dict[str, str]]