Cover fast-forward merge when the target tip is unresolved

What does this MR do and why?

Adds a spec for fast_forward! when the target branch tip can't be resolved. rspec:undercoverage flagged the nil branch of &.sha on the prior target sha lookup on !260375 (merged). The gap predates that MR. Spec only.

Detailed context for AI agents

• Uncovered line: app/services/merge_requests/merge_strategies/from_source_branch.rb:130, prior_target_sha = repository.commit(merge_request.target_branch_ref)&.sha. Undercover reported branches: 1/2 (80% method coverage). The nil branch was never exercised.
• Pre-existing: master has the same &.sha on the bare target_branch read since 13d415ea (removal of verify_ff_merge_advancement). !260375 (merged) only changed the argument to target_branch_ref, which put the method in the undercoverage diff. That MR carries pipeline:skip-undercoverage.
• New example: stubs repository.commit(target_branch_ref) to return nil and calls through for every other commit call. It asserts that ff_merge gets target_sha: nil and that the result is recorded.
• Verification: the new example passes. With &. replaced by ., it fails with NoMethodError, so it covers the nil branch. The full from_source_branch_spec.rb passes (29 examples), and rubocop is clean.
• No production code change, no changelog entry.

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