chore(build): include platform_tools in runtime wheel SUBPACKAGES

The PR-built wheel + import smoke gate refused the platform_tools
package because it's a new subdirectory under workspace/ that wasn't
in scripts/build_runtime_package.py:SUBPACKAGES. The drift gate (which
exists for exactly this reason) caught it cleanly:

  error: SUBPACKAGES drifted from workspace/ subdirectories:
    in workspace/ but NOT in SUBPACKAGES (will ship un-rewritten or
    be excluded): ['platform_tools']

Adding platform_tools to SUBPACKAGES wires the package into the
runtime wheel + applies the canonical
  from platform_tools.<x> -> from molecule_runtime.platform_tools.<x>
import-rewrite step that every other subpackage uses.

Verified locally: scripts/build_runtime_package.py succeeds, the
rewritten a2a_mcp_server.py reads
  from molecule_runtime.platform_tools.registry import TOOLS
which matches the package layout in the wheel.

scripts/build_runtime_package.py

@@ -83,6 +83,7 @@ SUBPACKAGES = {
     "adapters",
     "builtin_tools",
     "lib",
+    "platform_tools",
     "plugins_registry",
     "policies",
     "skill_loader",

workspace/a2a_mcp_server.py

@@ -13,7 +13,6 @@ Environment variables (set by the workspace container):
 """
 
 import asyncio
-import inspect
 import json
 import logging
 import sys