refactor(agents): consolidate chat runtime infra under chat/runtime

Move the lower-level runtime/infra modules out of multi_agent_chat/shared/
(they were never used by subagents, so they failed the shared-by-all-siblings
rule) and unify them with the already-relocated checkpointer:

  agents/runtime/                      -> agents/chat/runtime/
  mac/shared/errors.py                 -> chat/runtime/errors.py
  mac/shared/llm_config.py             -> chat/runtime/llm_config.py
  mac/shared/prompt_caching.py         -> chat/runtime/prompt_caching.py
  mac/shared/mention_resolver.py       -> chat/runtime/mention_resolver.py
  mac/shared/path_resolver.py          -> chat/runtime/path_resolver.py

These sit below the agent packages: the boundary + agent factory + shared
middleware depend on them, and they import no agent code (acyclic).

surfsense_backend/app/agents/chat/runtime/errors.py

@@ -0,0 +1,95 @@
+"""
+Typed error taxonomy for the SurfSense agent stack.
+
+Used by:
+- :class:`RetryAfterMiddleware` — its ``retry_on`` callable consults
+  the error code to decide whether a retry is appropriate.
+- :class:`PermissionMiddleware` — emits ``code="permission_denied"``
+  errors when a deny rule trips.
+- All tools — return :class:`StreamingError` payloads in
+  ``ToolMessage.additional_kwargs["error"]`` so the model and the
+  retry/permission layers share a contract.
+"""
+
+from __future__ import annotations
+
+from typing import Literal
+
+from pydantic import BaseModel, Field
+
+ErrorCode = Literal[
+    "rate_limit",
+    "auth",
+    "tool_validation",
+    "tool_runtime",
+    "context_overflow",
+    "provider",
+    "permission_denied",
+    "doom_loop",
+    "busy",
+    "cancelled",
+]
+
+
+class StreamingError(BaseModel):
+    """Structured error payload attached to ``ToolMessage.additional_kwargs["error"]``.
+
+    Tools and middleware emit this so retry, permission, and routing
+    layers can decide what to do without parsing free-form strings.
+    """
+
+    code: ErrorCode
+    retryable: bool = False
+    suggestion: str | None = None
+    correlation_id: str | None = None
+    detail: str | None = Field(
+        default=None,
+        description="Free-form additional context. Not surfaced to the model.",
+    )
+
+    class Config:
+        frozen = True
+
+
+class RejectedError(Exception):
+    """Raised when the user rejects a permission ask without feedback.
+
+    Caught by :class:`PermissionMiddleware`; the agent stops the current
+    tool fan-out and surfaces a user-facing rejection.
+    """
+
+    def __init__(self, *, tool: str | None = None, pattern: str | None = None) -> None:
+        super().__init__(f"Permission rejected for tool {tool!r}, pattern {pattern!r}")
+        self.tool = tool
+        self.pattern = pattern
+
+
+class CorrectedError(Exception):
+    """Raised when the user rejects a permission ask *with* feedback.
+
+    The :class:`PermissionMiddleware` translates the feedback into a
+    synthetic ``ToolMessage`` so the model sees the user's correction
+    and can retry the request differently.
+    """
+
+    def __init__(self, feedback: str, *, tool: str | None = None) -> None:
+        super().__init__(feedback)
+        self.feedback = feedback
+        self.tool = tool
+
+
+class BusyError(Exception):
+    """Raised when a second prompt arrives while the same thread is mid-stream."""
+
+    def __init__(self, request_id: str | None = None) -> None:
+        super().__init__("Thread is busy with another request")
+        self.request_id = request_id
+
+
+__all__ = [
+    "BusyError",
+    "CorrectedError",
+    "ErrorCode",
+    "RejectedError",
+    "StreamingError",
+]