chore: re-enable and fix the mypy delta presubmit

Scope the delta check to the files the CL changed via the kokoro
presubmit_request, and decouple the workspace copy so mypy module
resolution is stable.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 941321034

Author: GWeale

pyproject.toml

@@ -292,8 +292,11 @@ single_line_exclusions = []
 known_third_party = [ "a2a", "google.adk" ]
 
 [tool.mypy]
+mypy_path = [ "src" ]
 exclude = [ "contributing/samples/", "tests/" ]
-follow_imports = "skip"
+namespace_packages = true
+explicit_package_bases = true
+follow_imports = "normal"
 python_version = "3.11"
 disable_error_code = [ "import-not-found", "import-untyped", "unused-ignore" ]
 strict = true