Preserve federated quantiles across empty client digests

[sylvesterkaczmarek]

Fixes #5363.

Preserve the accumulated t-digest when a client supplies no digest, and replace an empty placeholder when a later client has data. This prevents client-order-dependent loss of quantiles and AttributeError from calling .merge() on an empty dictionary. All-empty features continue returning unavailable quantiles.

The regression exercises the public get_quantiles path using real fastdigest objects. It covers all six orders of two populated clients and one empty client, checks expected quantiles and unchanged serialized inputs, and retains an all-empty control.

Validation

python -m pytest -q tests/unit_test/app_common/statistics

92 passed on Linux ARM64, Python 3.12, with the project's pinned fastdigest==0.4.0. The focused regression on unchanged upstream gives 6 failures and 1 pass. Black, isort, flake8 and git diff --check pass for the changed files.

This was tested against the checkout's source in an isolated Linux container. A deployed federated job was not run.

[greptile-apps]

Confidence Score: 5/5

[Medium risk] Fixes quantile merging logic for empty client digests.

The PR appears safe to merge; no actionable issue was identified.

Summary

The PR preserves accumulated quantiles across empty client digests.

• Replaces an empty placeholder when a later client contributes a digest.
• Adds order-permutation and all-empty regression tests through get_quantiles.

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