Set mock search paths from the preprocessor's #includes (#1083 Case 1)

[mkarlesky]

Summary

This fixes a 1.2.0 prerelease regression, #1083 Case 1, and a second regression with the same cause.

A test's mock search paths came from stage 2's text scan of its #includes. That scan ignores #if, and the paths were never revisited after stage 4 settled the list through the preprocessor (introduced in 41602f17, first in v1.2.0-pre.8). Two things went wrong:

• A mock in an #if branch that is off, whose header does not exist, stopped the build in every mode (Found no file `Gpt_HwType1.h` ). On 1.1.10 it builds with :tests and :all.
• A mock only the preprocessor sees got no search path, such as one a macro names with its header in a subdirectory. Its generated mock was never found (mock_widget.h: No such file or directory). On 1.1.10 it builds.

Changes

• FileFinder#resolve_mock / #find_header_input_for_mock take complain:, defaulting to :error. With a tolerating level, a missing header resolves to [nil, nil]. Existing callers are unchanged.
• Stage 2 (collect_mock_search_paths) skips a scanned mock without a header when test preprocessing is on, because the preprocessor may discard it. With preprocessing off, the scanned list is the build's own, and a missing header still stops the build.
• Stage 4, first pass, where reconciliation follows, sets the mock search paths again from GCC's -MM -MG list, which is the only source reconciliation draws on. This happens before the directives-only pass, which needs those paths to find the stand-ins.
• Stand-in headers are now placed when the bare-includes list is recalled from cache, too, not only when it is extracted.
• lib/ceedling/test_invoker/PIPELINE.md is revised against the code: the two #include lists, stage 4's three passes, stand-ins, where mock search paths come from, and corrections throughout.

No Changelog entry, since this is a prerelease regression.

Testing

• Unit specs cover:
  • stage 2's tolerance and strictness;
  • pass 1 setting the paths from fresh and cached lists;
  • mocks the preprocessor discards never being resolved;
  • pass 2 receiving the rebuilt paths;
  • fallback mode leaving stage 2's paths in place;
  • stand-ins on a cache hit.
• New system spec, conditional_mock_includes:
  • Case 1 as reported passes with :tests and :all;
  • with :none, and with fallback preprocessing forced, the build stops naming the missing header, which pins the known gap;
  • the macro-named mock in a subdirectory passes with :tests and :all.
• Results:
  • rake specs:units and rake specs:integration pass, and lint is clean;
  • the new spec passes on the macOS host (Apple Clang) and in madsciencelab-plugins (GCC 14);
  • the related system specs pass in Docker.

#1083 stays open for Case 2, which needs its own design.

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