fix(mcp): harden project context issue provider and prove shared audit path

- Share cursor decoding in foundation.decode_offset_cursor so both tools
  reject an invalid cursor identically, before any upstream call.
- Add regression coverage that a malformed repo slug (owner/repo/extra,
  missing owner, empty segment) is refused before any network call.
- Add evidence that BOTH project context tools inherit the shared MCP
  client audit record and untrusted-content fence, instead of each tool
  shipping its own. No audit subsystem is duplicated.

mcp_servers/project_context/foundation.py

@@ -62,6 +62,23 @@ class ProviderError(RuntimeError):
         self.retryable = retryable
 
 
+def decode_offset_cursor(cursor: str | None) -> int:
+    """Shared opaque-cursor decoding for every paginated provider.
+
+    Rejected before any backend call so an invalid cursor never costs an
+    upstream request.
+    """
+    if cursor is None:
+        return 0
+    try:
+        offset = int(cursor)
+    except ValueError as exc:
+        raise ProviderError("INVALID_INPUT", "cursor is not valid.", retryable=False) from exc
+    if offset < 0:
+        raise ProviderError("INVALID_INPUT", "cursor is not valid.", retryable=False)
+    return offset
+
+
 ToolHandler = Callable[[ContractModel, Any], dict[str, Any]]
 
 

mcp_servers/project_context/providers/issue.py

@@ -16,7 +16,7 @@ from typing import Any, Protocol
 
 import requests
 
-from ..foundation import IdentityContext, ProviderError
+from ..foundation import IdentityContext, ProviderError, decode_offset_cursor
 
 # ---- tunables (documented, not hardcoded secrets) -------------------------
 _REQUEST_TIMEOUT_SECONDS = 10
@@ -203,7 +203,7 @@ class GiteaIssueProvider:
                 "issue_key must be a positive work item number.",
                 retryable=False,
             )
-        offset = self._decode_cursor(cursor)
+        offset = decode_offset_cursor(cursor)
 
         payload = self._fetch_issue(issue_key)
 
@@ -256,17 +256,6 @@ class GiteaIssueProvider:
         }
 
     # ---- internals ---------------------------------------------------
-    def _decode_cursor(self, cursor: str | None) -> int:
-        if cursor is None:
-            return 0
-        try:
-            offset = int(cursor)
-        except ValueError as exc:
-            raise ProviderError("INVALID_INPUT", "cursor is not valid.", retryable=False) from exc
-        if offset < 0:
-            raise ProviderError("INVALID_INPUT", "cursor is not valid.", retryable=False)
-        return offset
-
     def _build_description(self, body: str, detail: str) -> str:
         text = body.strip()
         if detail == "summary":