## Issue
Closes#5838
## Change
`GoogleGenAiToolMapper.convertToGoogleSchema` copied a
`JsonObjectSchema`'s properties into a `HashMap` and never set Gemini's
`propertyOrdering`. `JsonObjectSchema` keeps the declared order (backed
by a `LinkedHashMap`), but the copy lost it, so the generated schema's
field order was effectively arbitrary. Since Gemini uses
`propertyOrdering` to decide field emission order, structured-output
responses and function-call arguments could come back with fields in a
non-deterministic order.
Fix:
- copy properties into a `LinkedHashMap` to keep the declared order;
- set `propertyOrdering` to the property keys for objects with more than
one property.
This matches the official google-genai Python SDK (`_transformers.py`
sets `property_ordering = list(properties.keys())` for objects with >1
property) and the Vertex module in this repo (`SchemaHelper` already
uses a `LinkedHashMap`). The mapper is shared by function-call parameter
schemas and `responseSchema`, so both are fixed. The change is additive:
no public API change (revapi clean).
## Tests
Added
`should_preserve_declared_property_order_and_set_property_ordering` and
`should_set_property_ordering_on_nested_object` to
`GoogleGenAiToolMapperTest`. Both fail on `main` (the first because the
`HashMap` reorders the keys, the second because `propertyOrdering` is
absent) and pass with the fix.
```
mvn -pl langchain4j-google-genai verify -DskipITs # 133 unit tests, 0 failures; spotless + revapi green
mvn -pl langchain4j-core,langchain4j test # core 1270, 0 failures; main green
```
Both touched paths are also covered end-to-end by the module's live ITs
that run in CI: `GoogleGenAiAiServiceWithToolsIT` (function calling) and
`GoogleGenAiAiServiceWithJsonSchemaIT` (structured output).
## General checklist
- [x] There are no breaking changes (API, behaviour)
- [x] I have added unit and/or integration tests for my change
- [x] The tests cover both positive and negative cases
- [x] I have manually run all the unit and integration tests in the
module I have added/changed, and they are all green
- [x] I have manually run all the unit and integration tests in the core
and main modules, and they are all green
- [ ] I have added/updated the documentation
- [ ] I have added an example in the examples repo (only for "big"
features)
- [ ] I have added/updated Spring Boot starter(s) (if applicable)
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