ci: migrate from Travis to GitHub Actions

[woxxy]

Summary

This PR migrates CI from Travis to GitHub Actions and modernizes the local Docker test workflow used by CI.

What changed

• Added GitHub Actions workflow: .github/workflows/ci.yml
  • Runs on pull_request and pushes to master
  • Matrix mirrors prior coverage intent:
    • PHP: 7.4, 8.0, 8.1
    • Driver: mysqli, pdo
    • Search build: SPHINX2, SPHINX3, MANTICORE
  • SPHINX3 jobs are non-blocking (equivalent to prior Travis allow_failures)
  • Uploads debug artifacts on failure (tests/searchd.log, tests/searchd.pid, tests/data/*)
• Removed Travis config: .travis.yml
• Updated CI badge in README.md from Travis to GitHub Actions.
• Fixed tests/run.sh SPHINX3 UDF source resolution to be robust in CI (udfexample.c lookup no longer depends on $HOME/src).
• Added Docker-based test runner assets:
  • Dockerfile.test
  • scripts/run-tests-docker.sh
  • scripts/run-tests-in-container.sh
• Updated tests/README.md with Docker-only run instructions.

Reliability/Security improvements included

• Composer installer checksum verification in Dockerfile.test.
• Docker credential-helper fallback in runner script (retries build with isolated anonymous config only when helper is broken).
• Read-only source mount + temp workspace copy in container to avoid mutating local working tree.
• Modern PHP compatibility fixes in drivers to preserve existing test expectations on current runtimes.

Validation performed

• ./scripts/run-tests-docker.sh exits 0
• Both suites run:
  • Tests: 131, Assertions: 359, Risky: 3
  • Tests: 131, Assertions: 357, Risky: 3

Notes

• Existing risky tests and deprecation warnings are pre-existing and not introduced by this PR.
• After merge, ensure branch protection requires the new GitHub Actions checks (and does not require SPHINX3 checks).

[scrutinizer-notifier]

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