Stabilize mockgrbl e2e timing assertions

[ianbruene]

Motivation

• The existing Linux-only mockgrbl integration tests raced on asynchronous serial processing when reading /responses or accepting non-specific controller states, causing intermittent failures.
• The goal is to make the tests wait for observable mock output and require exact final controller state without changing production behavior or CI tags.

Description

• Add a bounded polling helper waitForMockResponses(t *testing.T, m *mockProcess, timeout time.Duration, pred func([]mockLogEntry) bool) []mockLogEntry that polls GET /responses and fails with useful diagnostics if the predicate is not met.
• Update the jog-limit rejection test in internal/integration/mockgrbl/mockgrbl_e2e_test.go to call waitForMockResponses(...) and wait for both [MSG:jogLIM] and error:15 before asserting the final safe mock state.
• Tighten the jog-completion test to require the controller-observed MachineState to be exactly Idle instead of accepting any non-Jog state, while keeping the existing final status request.
• Changes are limited to internal/integration/mockgrbl/mockgrbl_e2e_test.go and preserve all build tags and production behavior.

Testing

• Ran go test -tags 'serial mockgrbl_e2e' ./internal/integration/mockgrbl and it passed (ok github.com/ianbruene/ddgo/internal/integration/mockgrbl).
• Ran full unit suite with go test ./... and it passed for all packages.
• Ran targeted serial tests with go test -tags serial ./internal/transport ./internal/ports ./internal/app ./internal/grbl and they passed.
• Changed files: internal/integration/mockgrbl/mockgrbl_e2e_test.go.
• Commands executed: gofmt -w internal/integration/mockgrbl/mockgrbl_e2e_test.go, go test -tags 'serial mockgrbl_e2e' ./internal/integration/mockgrbl, go test ./..., and go test -tags serial ./internal/transport ./internal/ports ./internal/app ./internal/grbl.

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