fix: make strict-no-cover functional and add test pragma

Two issues prevented strict-no-cover from detecting unnecessary pragmas:

1. Coverage 7.13.x changed analysis_from_file_reporter to filter executed
   lines with '& statements', which removes excluded lines from the
   executed set. This breaks strict-no-cover's detection (it checks the
   intersection of excluded_lines and executed_lines). Pin to <7.13.

2. relative_files = true causes coverage to store relative paths, but
   strict-no-cover creates a temp rcfile without [tool.coverage.run]
   settings. Without relative_files in that temp config, coverage can't
   match paths, resulting in empty executed_lines.

Also adds a test pragma on a covered line to verify the check fails.

Author: maxisbey

pyproject.toml

@@ -67,7 +67,7 @@ dev = [
     "pytest-pretty>=1.2.0",
     "inline-snapshot>=0.23.0",
     "dirty-equals>=0.9.0",
-    "coverage[toml]>=7.13.1",
+    "coverage[toml]>=7.10.7,<7.13",
     "pillow>=12.0",
     "strict-no-cover",
 ]
@@ -201,7 +201,6 @@ branch = true
 patch = ["subprocess"]
 concurrency = ["multiprocessing", "thread"]
 source = ["src", "tests"]
-relative_files = true
 omit = [
     "src/mcp/client/__main__.py",
     "src/mcp/server/__main__.py",

src/mcp/shared/exceptions.py

@@ -13,7 +13,7 @@ class McpError(Exception):
     def __init__(self, error: ErrorData):
         """Initialize McpError."""
         super().__init__(error.message)
-        self.error = error
+        self.error = error  # pragma: no cover
 
 
 class StatelessModeNotSupported(RuntimeError):