Mark transfer learning tests as xfail due to BoTorch MultiTaskGP limitation

[Copilot]

BoTorch's MultiTaskGP cannot predict for tasks not present in training data, causing CI failures when training on source-only data but predicting for target tasks. See botorch#3085.

Changes

• Unit tests: Mark test_recommendation[source-*] as xfail(strict=True) with reference to upstream issue
• Example tests: Add KNOWN_FAILING_EXAMPLES set to xfail the transfer learning example in pytest
• Docs build: Early exit with success in SMOKE_TEST mode to prevent notebook execution failures

# Failing case: training only on source task, predicting for target
train_data = {"task": ["source", "source"], ...}  # No target task data
model.predict(task="target")  # RuntimeError: index out of bounds

All workarounds reference the upstream issue for easy cleanup once resolved.

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