#36252 ci, iwyu: skip subtrees in compilation database

• Stop Include What You Use CI checks from analyzing excluded subtree directories
• Avoid misleading warnings in CI logs and save runner time on third-party code

This pull request updates `ci/test/03_test_script.sh` to filter out shared excluded subtree directories (defined in `test/lint/lint_ignore_dirs.py`) from `compile_commands.json` before running Include What You Use (IWYU). This prevents IWYU from analyzing subtree source files like `libmultiprocess` while keeping `compile_commands.json` intact for tools like `clang-tidy`.

Problem: The IWYU CI job currently analyzes `src/ipc/libmultiprocess/`, generating warnings and suggested header edits that are discarded via `git restore`. This clutters CI job logs with misleading warnings and wastes CI runner time on third-party code maintained upstream.

P3 · cleanup

• P3 because it reduces CI log noise and redundant processing on subtrees
• Routine CI cleanup that does not fix a broken build or flaky test

P3 because it reduces CI log noise and redundant processing on subtrees. The author notes that '#35361 asked for subtrees not to be linted at all' and that IWYU suggestions for libmultiprocess disagree with upstream and are discarded anyway. It does not fix a broken build or flaky test, making it a routine CI maintenance cleanup.

Membership: Modifies `ci/test/03_test_script.sh` to adjust CI compilation database handling for IWYU.

Factors: security/stability 0, bug 1, performance 1, user value 0, leverage 0

• Ready to review
• Small 11-line CI script update with passing CI

The PR is an 11-line CI shell script addition with passing CI and no pending changes.

Author status: active

Resolved concerns:

• hebasto asked why subtrees are not excluded via CMAKE_EXPORT_COMPILE_COMMANDS in CMake; ryanofsky clarified that filtering in CI avoids breaking clang-tidy for subtrees and prevents build breakages with -DBUILD_TESTS=OFF.

• Neutral sentiment with no reviews or approvals yet
• Asked why subtrees are not excluded via CMake instead of CI (hebasto)
• Author explained CI filtering avoids breaking other tools like clang-tidy

Neutral; hebasto asked about a CMake-level alternative and ryanofsky provided a detailed rationale.

hebasto inquired about using CMake-level exclusion as attempted in #35418, to which the author explained the advantages of filtering at the CI step. No other reviews or ACKs have been posted yet.

• hebasto asked: 'Why not https://github.com/bitcoin/bitcoin/pull/35418#pullrequestreview-4428862711?'
• ryanofsky answered explaining that CI filtering avoids breaking other tools like clang-tidy and doesn't require modifying CMake subtree options.

Review verdicts (DrahtBot): 0

11 lines under test/bench/ci.

• ci/test/03_test_script.sh +11/-0

This PR strips subtree source files from compile_commands.json specifically during the IWYU step in ci/test/03_test_script.sh. It prevents IWYU from generating spurious header modification recommendations on upstream subtrees like libmultiprocess that get discarded at the end of the run. The change is small, self-contained, and follows up on issue #35361. hebasto inquired about doing the filtering in CMake instead, which the author answered with several practical reasons for doing it in the CI runner script.