test: mock git subprocess calls in create-pr.test.ts

[sonukapoor]

create-pr.test.ts runs real git commit calls during the test, which requires git user configuration to be present on the machine. On environments without a global git config (e.g. a fresh clone with no user.name/user.email), the test fails with:

Command failed: git commit -m "init"

The fix is to mock the git subprocess calls in the test rather than relying on a real git environment. The test should not depend on machine-level configuration to pass.

Related: #752 (proposes adding git config to CI as a workaround — the correct fix is in the test itself).