CI: make changelog automation LLM-based (Gemini) and race-safe

.github/pull_request_template.md

@@ -17,7 +17,7 @@
 - [ ] Docs have been reviewed and added / updated
 - [ ] Lint (`black rocketpy/ tests/`) has passed locally 
 - [ ] All tests (`pytest tests -m slow --runslow`) have passed locally
-- [ ] `CHANGELOG.md` has been updated (if relevant)
+- `CHANGELOG.md` — no action needed; an LLM workflow auto-updates it after merge
 
 ## Current behavior
 <!-- Describe current behavior or link to an issue. -->

.github/scripts/test_update_changelog.py

@@ -0,0 +1,179 @@
+#!/usr/bin/env python3
+"""Self-contained tests for update_changelog.py.
+
+Runs without network or the ``google-genai`` package (the LLM import in the
+script is lazy). Exercises the deterministic pieces that keep the changelog
+safe: section splitting, duplicate detection, prefix handling, the fallback
+classifier, and the validator that guards LLM output.
+
+Run locally with either:
+    python .github/scripts/test_update_changelog.py
+    pytest .github/scripts/test_update_changelog.py
+"""
+
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+
+import update_changelog as uc  # noqa: E402
+
+SAMPLE = """\
+# RocketPy Change Log
+
+## [Unreleased] - yyyy-mm-dd
+
+### Added
+
+### Changed
+
+### Fixed
+
+## [v1.13.0] - 2026-07-21
+
+### Added
+
+- ENH: something old [#100](https://github.com/RocketPy-Team/RocketPy/pull/100)
+"""
+
+
+def test_split_roundtrips():
+    before, block, after = uc.split_changelog(SAMPLE)
+    assert before + block + after == SAMPLE
+    assert block.startswith("## [Unreleased]")
+    assert "## [v1.13.0]" not in block
+    assert "## [v1.13.0]" in after
+
+
+def test_already_present():
+    _, block, after = uc.split_changelog(SAMPLE)
+    assert uc.already_present(after, 100) is True
+    assert uc.already_present(block, 100) is False
+    assert uc.already_present(block, 999) is False
+
+
+def test_detect_prefix():
+    assert uc.detect_prefix("BUG: fix a thing") == "BUG"
+    assert uc.detect_prefix("BUG/MNT: pre-release review fixes") == "BUG/MNT"
+    assert uc.detect_prefix("Add a shiny feature") is None
+
+
+def test_no_double_prefix():
+    # The historical bug: a title already carrying a prefix must not gain another.
+    entry = uc.build_entry("BUG/MNT: pre-release fixes", 1074, "ENH", "BUG/MNT")
+    assert entry.startswith("- BUG/MNT: pre-release fixes ")
+    assert "ENH:" not in entry
+    assert "[#1074](https://github.com/RocketPy-Team/RocketPy/pull/1074)" in entry
+
+
+def test_build_entry_adds_prefix_when_missing():
+    entry = uc.build_entry("Add a shiny feature", 200, "ENH", None)
+    assert entry.startswith("- ENH: Add a shiny feature ")
+
+
+def test_fallback_routing():
+    # Bug label -> Fixed
+    section, prefix, _ = uc.fallback_section_and_prefix("Fix crash", "Bug,Flight")
+    assert section == "### Fixed" and prefix == "BUG"
+    # Refactor label -> Changed
+    section, _, _ = uc.fallback_section_and_prefix("Tidy internals", "Refactor")
+    assert section == "### Changed"
+    # Existing prefix wins the routing even without a matching label
+    section, prefix, existing = uc.fallback_section_and_prefix("BUG: oops", "")
+    assert section == "### Fixed" and existing == "BUG"
+    # Default
+    section, prefix, _ = uc.fallback_section_and_prefix("New capability", "Enhancement")
+    assert section == "### Added" and prefix == "ENH"
+
+
+def test_fallback_update_inserts_once_under_right_section():
+    _, block, _ = uc.split_changelog(SAMPLE)
+    updated = uc.fallback_update(block, "BUG: fix a thing", 321, "Bug")
+    assert updated.count("[#321]") == 1
+    fixed = updated.split("### Fixed", 1)[1]
+    assert "- BUG: fix a thing" in fixed
+    # Not misplaced under Added.
+    added = updated.split("### Added", 1)[1].split("### Changed", 1)[0]
+    assert "[#321]" not in added
+
+
+def test_ensure_section_inserts_in_canonical_order():
+    _, block, _ = uc.split_changelog(SAMPLE)
+    # SAMPLE has Added/Changed/Fixed but no Removed subsection.
+    lines = uc.ensure_section(block.splitlines(keepends=True), "### Removed")
+    joined = "".join(lines)
+    # Removed must sit after Changed and before Fixed.
+    assert (
+        joined.index("### Changed")
+        < joined.index("### Removed")
+        < joined.index("### Fixed")
+    )
+
+
+def test_validator_accepts_good_output():
+    _, block, _ = uc.split_changelog(SAMPLE)
+    good = block.replace(
+        "### Fixed\n",
+        "### Fixed\n\n- BUG: fix it [#500](https://github.com/RocketPy-Team/RocketPy/pull/500)\n",
+    )
+    assert uc.validate_llm_block(block, good, 500) is None
+
+
+def test_validator_rejects_dropped_entry():
+    old = block_with_entry()
+    # New block loses the pre-existing entry.
+    new = "## [Unreleased] - yyyy-mm-dd\n\n### Fixed\n\n- BUG: new [#500](https://github.com/RocketPy-Team/RocketPy/pull/500)\n\n"
+    assert uc.validate_llm_block(old, new, 500) is not None
+
+
+def test_validator_rejects_missing_new_ref():
+    _, block, _ = uc.split_changelog(SAMPLE)
+    assert uc.validate_llm_block(block, block, 500) is not None  # no new ref at all
+
+
+def test_validator_rejects_leaked_version_header():
+    _, block, _ = uc.split_changelog(SAMPLE)
+    leaked = (
+        "## [Unreleased] - yyyy-mm-dd\n\n### Fixed\n\n"
+        "- BUG: x [#500](https://github.com/RocketPy-Team/RocketPy/pull/500)\n\n"
+        "## [v1.13.0] - 2026-07-21\n"
+    )
+    assert uc.validate_llm_block(block, leaked, 500) is not None
+
+
+def test_validator_rejects_runaway_growth():
+    _, block, _ = uc.split_changelog(SAMPLE)
+    huge = (
+        "## [Unreleased] - yyyy-mm-dd\n\n### Fixed\n\n"
+        "- BUG: x [#500](https://github.com/RocketPy-Team/RocketPy/pull/500)\n"
+        + ("x" * (uc.MAX_BLOCK_GROWTH + 50))
+    )
+    assert uc.validate_llm_block(block, huge, 500) is not None
+
+
+def block_with_entry():
+    return (
+        "## [Unreleased] - yyyy-mm-dd\n\n### Added\n\n"
+        "- ENH: keep me [#200](https://github.com/RocketPy-Team/RocketPy/pull/200)\n\n"
+        "### Fixed\n\n"
+    )
+
+
+def _run_all():
+    tests = [
+        v for k, v in sorted(globals().items()) if k.startswith("test_") and callable(v)
+    ]
+    failures = 0
+    for test in tests:
+        try:
+            test()
+            print(f"  ok   {test.__name__}")
+        except AssertionError as exc:
+            failures += 1
+            print(f"  FAIL {test.__name__}: {exc}")
+    print(f"\n{len(tests) - failures}/{len(tests)} passed")
+    return 1 if failures else 0
+
+
+if __name__ == "__main__":
+    sys.exit(_run_all())