chore(gitignore): anchor the mock-backend pattern so its source dir is traversable

The bare `mock-backend` pattern matched the *directory*
tests/e2e/mock-backend/, not just the binary built into it. Git will not
descend into an ignored directory even for tracked files, so
`git add tests/e2e/mock-backend/main.go` required -f. This was hit while
working on #10970.

Anchor it to the artifact's full path. The built binary stays ignored (it is
also covered by tests/e2e/mock-backend/.gitignore) while the source directory
becomes traversable again.

Verified with `git check-ignore -v`: a new source file under
tests/e2e/mock-backend/ is no longer ignored, and the binary produced by
`make build-mock-backend` still is.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: Claude:claude-opus-4-8 [Claude Code]

Author: mudler

.gitignore

@@ -41,7 +41,12 @@ models/*
 test-models/
 test-dir/
 tests/e2e-aio/backends
-mock-backend
+# The mock backend binary built by `make build-mock-backend`. Anchored to its
+# full path: a bare `mock-backend` also matched the *directory* holding the
+# source, so git would not descend into it and adding a file there needed -f.
+# tests/e2e/mock-backend/.gitignore covers the same binary; kept here too so
+# the artifact stays ignored if that scoped file is ever removed.
+/tests/e2e/mock-backend/mock-backend
 
 release/