Add serialization snapshot tests and backward-compat deserialization tests

[andrewherren]

Part of #315 (cross-version deserialization, RFC 0002).

Two categories of tests, in both R (test/R/testthat/) and Python (test/python/):

1. Snapshot tests
Serialize a BART and BCF model (covering: RFX, adaptive coding, variance forest, internal propensity) under the current version and save the JSON as a checked-in fixture. Future test runs load the fixture and assert deserialization succeeds — preventing silent regressions when the schema changes.

2. Backward-compat tests
Construct minimal "old-format" JSON strings with specific fields deliberately omitted (mirroring the version brackets in #318) and assert that:

• Deserialization succeeds (no error)
• The correct default value is applied
• The expected warning is emitted (use expect_warning / pytest.warns)

Fields to cover at minimum:

• Missing outcome / link (pre-v0.4.1 format)
• Missing multivariate_treatment (pre-v0.4.0 Python BCF)
• Missing internal_propensity_model (pre-v0.3.2)
• Missing rfx_model_spec with has_rfx = TRUE (should warn)
• Missing covariate_preprocessor / preprocessor_metadata (should warn)