Stop the #854 shim warning on causalml's own internal fit calls

[jeongyoonlee]

Proposed changes

Follow-up to #975. The deprecation shim suppresses re-entrant warnings with a flag stored on the instance, so it covers self.fit → self.predict nesting but not the forest → tree hop: the flag is set on the forest while the warning fires on each tree it fits.

The result is a warning the caller cannot act on or silence:

>>> CausalRandomForestRegressor(n_estimators=2).fit(X=X, y=y, treatment=treatment)
FutureWarning: Passing `treatment` and/or `y` to fit() by position is deprecated ...
FutureWarning: Passing `treatment` and/or `y` to fit() by position is deprecated ...

That call is all-keyword — written exactly the way the warning asks for — and it emits one FutureWarning per tree: 100 at the default n_estimators. UpliftRandomForestClassifier behaves the same way (default 10).

This makes the forests pass y and treatment to their trees by keyword. It is the same migration #982 applies to tests/, applied to the library's own call sites — the epic (#980) has no ticket covering those.

Call sites changed

File
causalml/inference/tree/causal/causalforest.py:136	subsample / bootstrap branch
causalml/inference/tree/causal/causalforest.py:145	non-subsample branch
causalml/inference/tree/_uplift/upliftforest.py:73	bagged uplift tree fit

Verification

An empirical probe over eleven public entry points (both forests, both trees, the S/T/X/R/DR learners, estimate_ate, fit_predict), all called with keywords, attributes each warning to its originating library line via stacklevel=2: 2 offending sites before, 0 after.

• tests/test_fit_arg_order.py: 54 passed.
• tests/test_causal_trees.py tests/test_uplift_trees.py tests/test_uplift_trees_kernel.py tests/test_serialization_extended.py: 135 passed.
• Both new regression tests fail with the library change reverted and pass with it, so they pin the fix rather than passing vacuously.
• black --check clean.

Left alone, deliberately

The same-instance super().fit(X, treatment, y, ...) calls in upliftforest.py:342, uplifttree.py:567 and rlearner.py:913. The guard already covers them and they warn nobody today. They do need to change under #985 — once the signature order flips, treatment silently lands in y.

Note on why this was not caught

The repo sets no filterwarnings anywhere, so FutureWarning is never an error and the affected tests pass on a normal run. A deprecation shim without a warnings-as-errors lane is untested by construction; worth pairing with the CI work in #977.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

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[paullo0106]

lgtm