fix: add vLLM/local server error patterns + MCP initial connection retry (#9281)

Port two improvements inspired by Kilo-Org/kilocode analysis:

1. Error classifier: add context overflow patterns for vLLM, Ollama,
   and llama.cpp/llama-server. These local inference servers return
   different error formats than cloud providers (e.g., 'exceeds the
   max_model_len', 'context length exceeded', 'slot context'). Without
   these patterns, context overflow errors from local servers are
   misclassified as format errors, causing infinite retries instead
   of triggering compression.

2. MCP initial connection retry: previously, if the very first
   connection attempt to an MCP server failed (e.g., transient DNS
   blip at startup), the server was permanently marked as failed with
   no retry. Post-connect reconnection had 5 retries with exponential
   backoff, but initial connection had zero. Now initial connections
   retry up to 3 times with backoff before giving up, matching the
   resilience of post-connect reconnection.
   (Inspired by Kilo Code's MCP server disappearing fix in v1.3.3)

Tests: 6 new error classifier tests, 4 new MCP retry tests, 1
updated existing test. All 276 affected tests pass.

tools/mcp_tool.py

@@ -162,6 +162,7 @@ if _MCP_AVAILABLE and not _MCP_MESSAGE_HANDLER_SUPPORTED:
 _DEFAULT_TOOL_TIMEOUT = 120      # seconds for tool calls
 _DEFAULT_CONNECT_TIMEOUT = 60    # seconds for initial connection per server
 _MAX_RECONNECT_RETRIES = 5
+_MAX_INITIAL_CONNECT_RETRIES = 3 # retries for the very first connection attempt
 _MAX_BACKOFF_SECONDS = 60
 
 # Environment variables that are safe to pass to stdio subprocesses
@@ -984,6 +985,7 @@ class MCPServerTask:
                 self.name,
             )
         retries = 0
+        initial_retries = 0
         backoff = 1.0
 
         while True:
@@ -997,11 +999,37 @@ class MCPServerTask:
             except Exception as exc:
                 self.session = None
 
-                # If this is the first connection attempt, report the error
+                # If this is the first connection attempt, retry with backoff
+                # before giving up. A transient DNS/network blip at startup
+                # should not permanently kill the server.
+                # (Ported from Kilo Code's MCP resilience fix.)
                 if not self._ready.is_set():
-                    self._error = exc
-                    self._ready.set()
-                    return
+                    initial_retries += 1
+                    if initial_retries > _MAX_INITIAL_CONNECT_RETRIES:
+                        logger.warning(
+                            "MCP server '%s' failed initial connection after "
+                            "%d attempts, giving up: %s",
+                            self.name, _MAX_INITIAL_CONNECT_RETRIES, exc,
+                        )
+                        self._error = exc
+                        self._ready.set()
+                        return
+
+                    logger.warning(
+                        "MCP server '%s' initial connection failed "
+                        "(attempt %d/%d), retrying in %.0fs: %s",
+                        self.name, initial_retries,
+                        _MAX_INITIAL_CONNECT_RETRIES, backoff, exc,
+                    )
+                    await asyncio.sleep(backoff)
+                    backoff = min(backoff * 2, _MAX_BACKOFF_SECONDS)
+
+                    # Check if shutdown was requested during the sleep
+                    if self._shutdown_event.is_set():
+                        self._error = exc
+                        self._ready.set()
+                        return
+                    continue
 
                 # If shutdown was requested, don't reconnect
                 if self._shutdown_event.is_set():