test(train): make SFT recipe-override integ tests recipe-proof

The canaries-v3-master (sagemaker-train) job started failing after the
JumpStart Hub SFT recipe for meta-textgeneration-llama-3-2-1b-instruct
was updated (hub content 2.17.0 / template v1.0.3): training_args
nesting was removed, max_len renamed to dataset_max_len, the default
learning_rate changed, and some validation bounds were relaxed. The SFT
recipe-override integ tests pinned the old recipe shape/values, so they
broke with no SDK or test change.

Remove the 15 structure-pinning SFT tests; their merge/override/flatten/
validation behavior is already covered Hub-independently by the pinned-
fixture unit tests in tests/unit/train/test_recipe_resolver.py. Add a
schema-agnostic llama-SFT serverless test mirroring the existing RLVR and
Nova serverless tests: keys are discovered dynamically from the live Hub
spec and overrides are asserted by value, so the test survives future
recipe/version drift. Leave the already recipe-proof suites (ModelTrainer
local-recipe, evaluator, RLVR/Nova serverless) untouched.

Author: rsareddy0329