Add regression test: RampedHalfAndHalf initializer does not respect max_depth (#259)

[alcides]

Summary

Adds a strict-xfail regression test documenting issue #259: RampedHalfAndHalfInitializer accepts a max_depth argument but does not respect it.

No production code is changed in this PR — the test is living evidence of the bug, and will start failing (via xfail(strict=True)) the moment the bug is fixed, prompting removal of the marker.

Root cause (for reference)

RampedHalfAndHalfInitializer.__init__ stores max_depth, but initialize() calls representation.create_genotype(random) without passing a depth-bounded decider, so the representation's default decider is used and max_depth is silently ignored (compare with FullInitializer, which passes its own depth-bounded FullDecider). It also does not do any ramping or grow/full alternation, contrary to its docstring.

How to run

uv run pytest tests/representations/tree_based/initializer_test.py::TestInitializers::test_ramped_half_and_half_respects_max_depth -v

Currently reports XFAIL. To see the underlying failure, remove the xfail marker and run again.

Evidence (assertion output without the xfail marker)

The test builds a tiny recursive grammar (Expr → Leaf | Branch(Expr, Expr)), requests a population of 20 from RampedHalfAndHalfInitializer(max_depth=3) with a fixed seed (NativeRandomSource(5)), and asserts every individual's depth is ≤ 3:

E       AssertionError: RampedHalfAndHalfInitializer(max_depth=3) produced trees with depths [9, 3, 3, 3, 4, 1, 1, 1, 1, 1, 1, 1, 5, 2, 1, 1, 1, 1, 6, 5]
E       assert False

Expected: all depths ≤ 3. Actual: 5 of 20 individuals exceed the limit (depths 4, 5, 5, 6, and 9).

Checks

• uv run pytest: 520 passed, 8 skipped, 1 xfailed (the new test)
• ruff check . --fix: all checks passed

Closes nothing; references #259.