Improve coverage of `exec()` package-loading error paths

[mcriley821]

Summary

exec() (simplemock.go:160-341) contains several error branches that are only reachable by running the full binary and are not covered by any focused test. The package-loading section is the hardest to reach via E2E because it requires simulating failures in the Go toolchain or type-checker.

Uncovered error paths

Line	Condition
203–206	packages.Load() returns a non-nil error
207–210	packages.Load() returns an empty package list
212–219	Loaded package has parse errors or type errors (pkg.Errors)
221–224	filepath.Abs() fails and the fallback path is used
258–261	Type-checker lookup succeeds but the object is not a *types.TypeName

Proposed approach

Refactor exec() to accept a loadFunc parameter (or similar interface) so that packages.Load can be swapped for a stub in tests. This would allow each error branch to be triggered without needing a full Go toolchain or real source files.

Alternatively, add integration-style tests that set up minimal on-disk Go packages in t.TempDir() and invoke exec() directly.