Test-build pipeline Phase B, PR 1: mechanical cleanups and small consolidations

[mkarlesky]

Summary

First of four PRs implementing Phase B of the test-build pipeline (lib/ceedling/test_invoker/) architecture review, following PR #1217 (Phase A). This PR bundles the lowest-risk, non-overlapping items -- all behavior-preserving, none touching DI wiring or adding a new class.

• Simplify stage_flatten_objects_list's nested map { ... }.flatten to flat_map.
• Rename run_fixture_now to run_fixture (same historical _now Rake-extraction-artifact pattern already resolved for generate_executable_now in Phase A).
• Extract three repeated literals into shared TestPipelineHelpers methods: the vendor-path array literal (11 call sites), the { flags:, defines:, search_paths: } meta-hash (6 call sites), and (private to TestBuildExecutor) the directives-only fallback condition (4 call sites).
• Replace objects_list's raw { test:, obj: } Hash entries with a named ObjectWork struct, matching PartialWork/MockWork's existing pattern for the sibling T1/T2 transforms.
• Consolidate the identical TestBuildSetup#mock_partial?/TestBuildPlanner#is_mock_partial? predicates into one shared TestPipelineHelpers method.
• Change Partializer#extract_module_contents's boolean-trap third positional arg to a keyword arg (fallback:), and bring its doc comment back in line with the method's actual current signature.
• Extract tailor_search_paths's four branches into named private methods for readability, keeping the if/elsif dispatch itself (a true Hash-dispatch table would need a Hash of Procs here, not a Hash of static values -- not a good fit for this method's config-gated, priority-ordered matching).

Test plan

• bundle exec rspec spec/units -- 2588 examples, 0 failures
• Each of the 9 commits verified independently against its own touched spec files before the next commit landed

No targeted Docker system-spec run for this PR -- every change here is either behavior-preserving with strong existing unit coverage, or (the extract_module_contents keyword-arg change) touches Partializer with its own strong existing spec. Relying on CI for system coverage.

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