feat(ci): Add file change detection to workflows

[safina57]

Description

Adds a files-changed job to the test orchestrator that detects whether a PR touches anything beyond docs (markdown, docs/, LICENSE, .agents/, issue templates, CODEOWNERS). Each test group, and the coverage report job, now needs it and only runs when code changed. Docs-only PRs skip every group; check-required still reports, so the required status check keeps working. Pushes to master and release branches always run the full suite.

Tested on my fork with mock PRs: a CHANGELOG.md-only change skipped all groups and the coverage report while "All tests passed" stayed green, and a pyproject.toml-only change ran the full suite.

Issues

• resolves: Do not run tests when only documentation is changed #4859

[alexander-alderman-webb]

This won't work because the tests are required jobs.

[safina57]

@alexander-alderman-webb the only test-related required check is All tests passed, not the individual groups and that job keeps if: always() and isn't gated on files-changed, so it always runs and reports, skipped groups don't count as failure/cancelled, so it goes green on docs-only PRs. The report job is gated too so it doesn't try to combine non-existent coverage, verified this by making a change in a **.md file where PR skipped all groups and All tests passed succeeded, https://github.com/safina57/sentry-python/actions/runs/35128371710, and another one where I made a change on pyproject.toml and the gate worked as intended https://github.com/safina57/sentry-python/actions/runs/35128878739/job/104904614766