overlays: work around nixpkgs#578265 (posthog broken with mcp 2)

nixpkgs#575004 bumped mcp 1.29 -> 2.2 and marked python3Packages.posthog
broken; only one optional MCP test fails. frappe needs posthog at runtime.
Skip that test and clear meta.broken until the upstream unbreak lands;
the overlay warns at eval time once it does.

overlays/default.nix

@@ -2,8 +2,8 @@
 {
   flake.overlays = {
     packagesOverlay = import ../pkgs/overlay.nix;
-    # TEMPORARY: drop with the file once NixOS/nixpkgs#576853 reaches nixpkgs-unstable.
+    # TEMPORARY: drop with the file once NixOS/nixpkgs#578265 reaches nixos-unstable.
-    phpPr576853Overlay = import ./nixpkgs-pr-576853-php.nix;
+    posthogPr578265Overlay = import ./nixpkgs-pr-578265-posthog.nix;
 
     frappixFrappeOverlay = inputs.frappix.frappeOverlay.x86_64-linux;
     frappixLibsOverlay = inputs.frappix.libsOverlay.x86_64-linux;

overlays/nixpkgs-pr-578265-posthog.nix

@@ -0,0 +1,33 @@
+# TEMPORARY workaround for NixOS/nixpkgs#578265 (python3Packages.posthog: unbreak).
+# nixpkgs#575004 bumped mcp 1.29 -> 2.2 and marked posthog broken; the only
+# failure is one test in posthog's optional MCP integration (field rename in
+# mcp 2). posthog is a runtime dependency of frappe, so frappix machines fail
+# to evaluate without this.
+#
+# Remove once `nix flake update` pulls the PR in: delete this file and every
+# reference to it (grep 578265). The overlay becomes a no-op and warns at eval
+# time as soon as the locked nixpkgs no longer marks posthog broken.
+_final: prev:
+let
+  inherit (prev) lib;
+in
+{
+  pythonPackagesExtensions = prev.pythonPackagesExtensions ++ [
+    (_pyFinal: pyPrev: {
+      posthog =
+        if !pyPrev.posthog.meta.broken then
+          lib.warn "nixpkgs no longer marks posthog broken: NixOS/nixpkgs#578265 has landed, remove overlays/nixpkgs-pr-578265-posthog.nix" pyPrev.posthog
+        else
+          pyPrev.posthog.overridePythonAttrs (old: {
+            disabledTests = old.disabledTests ++ [
+              # AttributeError: 'CallToolResult' object has no attribute 'structuredContent'
+              # (mcp>=2 renamed the field to `structured_content`)
+              "test_prepare_tool_result_delivers_into_call_tool_result_models"
+            ];
+            meta = old.meta // {
+              broken = false;
+            };
+          });
+    })
+  ];
+}