Skip writing default attribute values in IIDM serialization

[clementleclercRTE]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• A PR or issue has been opened in all impacted repositories (if any)

Does this PR already have an issue describing the problem?

What kind of change does this PR introduce?

Performance feature: reduction of serialized file size by omitting attributes equal to their default value.

What is the current behavior?

Every attribute is always written during serialization, even when its value equals the defined default. This produces unnecessarily verbose output, especially for tap changer steps (4 attributes
r, x, g, b almost always zero), switches (retained/open typically false), and transformer/line shunt parameters (g, b typically zero). On large networks this has a measurable impact on file size
and I/O performance, particularly for the binary serialization format.

What is the new behavior (if this is a feature change)?

Attributes whose value equals their default are omitted from the serialized output. The read side is unaffected: all readers already use readDoubleAttribute(name,defaultValue) / readBooleanAttribute(name, defaultValue), which transparently return the default when an attribute is absent.
Versions ≤ 1.16 are unchanged — all attributes continue to be written unconditionally, preserving full backward compatibility.
For the binary format, this PR requires PR #3804, which makes the binary reader order-independent and tolerant of absent attributes.

Full list of attributes now omitted when equal to their default :

Attribute	Type	Changed to
retained	Switch	optional default false
open	Switch	optional default false
r, x, g, b	RatioTapChangerStep	optional default 0
r, x, g, b	PhaseTapChangerStep	optional default 0
lowTapPosition	RatioTapChanger, PhaseTapChanger	optional default 0
loadTapChangingCapabilities	RatioTapChanger only	optional default false
minP	Generator	optional default 0
voltageRegulatorOn	Generator	optional default false
g1, g2, b1, b2	Line	optional default 0
g, b	TwoWindingsTransformer	optional default 0

Does this PR introduce a breaking change or deprecate an API?

• Yes
• No

If yes, please check if the following requirements are fulfilled

• The Breaking Change or Deprecated label has been added
• The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

Other information:

The internal method writeTapChangerStep in AbstractTransformerSerDe had its signature changed from (TapChangerStep, TreeDataWriter) to (TapChangerStep, NetworkSerializerContext) in order to access
the IIDM version inside that method for the version guard. This is a package-private refactoring with no impact on the public API.