[9.1] (backport #18386) Backport smart exhaustive tests

[mergify]

Backports the following commits:

bf89f43 (HEAD, upstream/main) chore: remove test file for exhaustive tests and normalise github commands (#18384)
02aa5e1 bk(smart exhaustive tests): tune the steps to support GH comments and changesets (#18383)
be64ebf bk: fix regex and add new file (#18382)
fc9ad7f feat(ci): support exhaustive tests for PRs (code changes or GH comment) (#18327)

To solve:

https://buildkite.com/elastic/logstash-smart-exhaustive-tests-pipeline/builds/32#019a3b6c-873e-4817-8fe1-4552c0fba68a

2025-10-31 10:59:54 PDT
Running agent environment hook
2025-10-31 10:59:56 PDT
Preparing working directory
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Running commands
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$ buildkite-agent pipeline upload .buildkite/smart_exhaustive_tests_pipeline.yml
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2025-10-31 18:00:00 INFO   Reading pipeline configs from [".buildkite/smart_exhaustive_tests_pipeline.yml"]
2025-10-31 11:00:00 PDT
buildkite-agent: fatal: failed to read file: open .buildkite/smart_exhaustive_tests_pipeline.yml: no such file or directory
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🚨 Error: The command exited with status 1
2025-10-31 11:00:00 PDT
user command error: exit status 1
```<hr>This is an automatic backport of pull request #18386 done by [Mergify](https://mergify.com).

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)
• /run exhaustive tests : Run the exhaustive tests Buildkite pipeline.

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: d123f03

cc @donoghuc