refactor(server): keep ResourceNotFoundError out of top-level mcpserver exports

Per review, MCPServer's top-level __all__ doesn't expose exception types.
Template handlers that want to signal SEP-2164 not-found should import
ResourceNotFoundError from mcp.server.mcpserver.exceptions (alongside
ToolError). The template pass-through and behaviour are unchanged.

Also clarifies the ResourceNotFoundError docstring to describe the
template-handler contract.

Author: pja-ant

docs/migration.md

@@ -345,7 +345,7 @@ The internal layers (`ToolManager.call_tool`, `Tool.run`, `Prompt.render`, `Reso
 
 ### `ResourceManager.get_resource()` and `ResourceTemplate.create_resource()` raise typed exceptions
 
-`ResourceManager.get_resource()` now raises `ResourceNotFoundError` (instead of `ValueError`) when no resource or template matches the URI. `ResourceTemplate.create_resource()` now raises `ResourceError` (instead of `ValueError`) when the template function fails. Neither subclasses `ValueError`, so callers catching `ValueError` should switch to `ResourceNotFoundError` / `ResourceError` (both importable from `mcp.server.mcpserver`). `MCPServer.read_resource()` continues to raise `ResourceError` and is unaffected.
+`ResourceManager.get_resource()` now raises `ResourceNotFoundError` (instead of `ValueError`) when no resource or template matches the URI. `ResourceTemplate.create_resource()` now raises `ResourceError` (instead of `ValueError`) when the template function fails. Neither subclasses `ValueError`, so callers catching `ValueError` should switch to `ResourceNotFoundError` / `ResourceError` (both importable from `mcp.server.mcpserver.exceptions`). `MCPServer.read_resource()` continues to raise `ResourceError` and is unaffected.
 
 ### Replace `RootModel` by union types with `TypeAdapter` validation
 

src/mcp/server/mcpserver/__init__.py

@@ -3,8 +3,7 @@
 from mcp.types import Icon
 
 from .context import Context
-from .exceptions import ResourceError, ResourceNotFoundError
 from .server import MCPServer
 from .utilities.types import Audio, Image
 
-__all__ = ["MCPServer", "Context", "Image", "Audio", "Icon", "ResourceError", "ResourceNotFoundError"]
+__all__ = ["MCPServer", "Context", "Image", "Audio", "Icon"]

src/mcp/server/mcpserver/exceptions.py

@@ -14,7 +14,12 @@ class ResourceError(MCPServerError):
 
 
 class ResourceNotFoundError(ResourceError):
-    """Resource does not exist."""
+    """Resource does not exist.
+
+    Raise this from a resource template handler to signal that the requested
+    instance does not exist; clients receive ``-32602`` (invalid params) per
+    SEP-2164.
+    """
 
 
 class ToolError(MCPServerError):

tests/server/mcpserver/test_server.py

@@ -12,8 +12,8 @@
 from mcp.client import Client
 from mcp.server.context import ServerRequestContext
 from mcp.server.experimental.request_context import Experimental
-from mcp.server.mcpserver import Context, MCPServer, ResourceNotFoundError
-from mcp.server.mcpserver.exceptions import ToolError
+from mcp.server.mcpserver import Context, MCPServer
+from mcp.server.mcpserver.exceptions import ResourceNotFoundError, ToolError
 from mcp.server.mcpserver.prompts.base import Message, UserMessage
 from mcp.server.mcpserver.resources import FileResource, FunctionResource
 from mcp.server.mcpserver.utilities.types import Audio, Image