Let a reference sample inherit the expected values of its definition

[ramonski]

Description of the issue/feature this PR addresses

A reference sample points at the reference definition it was bought against. The definition carries the expected values, and those values decide which analysis services the sample supports.

Current behavior before PR

The values are copied from the definition to the sample by the edit form adapter, in the browser, on the modified event of the ReferenceDefinition field (senaite/core/browser/form/adapters/referencesample.py). Nothing on the server does it.

A reference sample created any other way therefore links a definition and holds no expected values:

• over the JSON API
• from a setup data import
• by api.create
• by copying one in the ZMI

getSupportedServices reads the expected values, so such a sample supports no service. On a worksheet it is listed under Add Blank / Add Control with an empty Supported Services column, and selecting it adds no reference analysis. The sample looks complete everywhere else: it has a definition, a lot number, an expiry date.

Desired behavior after PR is merged

ReferenceSample.getReferenceResults falls back to the definition when the sample holds no values of its own. Values set on the sample still win, and clearing them hands the sample back to its definition.

This is the behaviour getHazardCategories in the same class already has, twenty lines further down, and for the same reason.

Nothing about the edit form changes. A sample saved through it keeps its own copy of the values, as before.

Verification

A new doctest, ReferenceSampleFromDefinition.rst, covers the three cases: a sample that inherits, a sample whose own values win, and a sample with neither a definition nor values of its own.

Without the fix it fails on the first assertion; with it, it passes.

Run alongside the reference and QC doctests that read the same accessor, all green:

ReferenceSampleFromDefinition             Ran 1 tests with 0 failures
WorksheetAddReferenceSamples              Ran 1 tests with 0 failures
WorkflowReferenceAnalysisBlankSubmit      Ran 1 tests with 0 failures
WorkflowReferenceAnalysisControlSubmit    Ran 1 tests with 0 failures
QCAnalysesWithInterimFieldsOnAWorksheet   Ran 1 tests with 0 failures
API_analysis                              Ran 2 tests with 0 failures
InstrumentResultsImport                   Ran 1 tests with 0 failures

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I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.