### What changes were proposed in this PR?

Dendrogram's Color Threshold could not be set at all:

- the field was declared as a string and spliced into the generated
Python as a decode expression, so scipy received `'3'` rather than `3`
and raised `UFuncTypeError` comparing it against the linkage distances
- the only values that plotted were a blank field and the literal
`default`, and scipy documents those as equivalent — both mean 0.7 × max
distance — so nothing a user could type ever changed the coloring

This PR declares the field as `Option[Double]`:

- the number is spliced as a literal, so scipy gets a number
- an unset threshold stays `None`, the same 0.7 × max distance a blank
field already meant
- `@JsonDeserialize(contentAs = ...)` names the boxed class: Scala
erases `Option`'s element type, so without it Jackson leaves the raw
JSON value inside the Option and the first use throws
`ClassCastException`, and the primitive class would read a blank as 0 —
every link colored the same rather than "unset"

Compatibility: a numeric string saved earlier still reads as a number,
and those workflows were failing before this change anyway. A workflow
that stored the literal `default` no longer loads; clearing the field
plots the identical chart.

### Any related issues, documentation, discussions?

Fixes #7232.

### How was this PR tested?

- the operator's existing spec updated: a configured threshold is
asserted to reach the template as `color_threshold=42.5`, not as a
decoded string
- deserialization tests for a JSON number, a numeric string, blank, null
and absent — plus one that uses the value as a number, the case a round
trip cannot catch
- ran the generated Python against a pandas DataFrame:
`color_threshold=3.0` plots a figure and so does the unset case, while
passing the same value as a string, which is what the operator does
today, raises `UFuncTypeError`
- whole workflow-operator module: 2020 tests passing, `scalafmtCheck`
clean

### Was this PR authored or co-authored using generative AI tooling?

(backported from commit 351ce20)

Generated-by: Claude Code (claude-opus-5[1m])