fix: chain exceptions in get_prompt and read_resource handlers

Two raise statements discarded the causal exception chain:

- Line 1112: raise ValueError(str(e)) → raise ValueError(str(e)) from e
- Line 451: raise ResourceError(...) → raise ResourceError(...) from exc

The same file already uses `from exc` correctly at line 459.
Without `from`, callers lose the original traceback and cannot
programmatically inspect the root cause via __cause__.

Fixes #2541

Author: blackwell-systems

src/mcp/server/mcpserver/server.py

@@ -447,8 +447,8 @@ async def read_resource(
             context = Context(mcp_server=self)
         try:
             resource = await self._resource_manager.get_resource(uri, context)
-        except ValueError:
-            raise ResourceError(f"Unknown resource: {uri}")
+        except ValueError as exc:
+            raise ResourceError(f"Unknown resource: {uri}") from exc
 
         try:
             content = await resource.read()
@@ -1109,4 +1109,4 @@ async def get_prompt(
             )
         except Exception as e:
             logger.exception(f"Error getting prompt {name}")
-            raise ValueError(str(e))
+            raise ValueError(str(e)) from e