Preserve unset booleans in GoogleGenAiChatOptions

[dlwldn30]

Problem

GoogleGenAiChatOptions converts unset googleSearchRetrieval and includeServerSideToolInvocations values from null to false when options are built.

ChatClient merges request options into model defaults by applying only non-null request values. Because an unrelated request option is already converted to false, it unintentionally disables model-level Google Search and server-side tool invocation defaults.

Changes

• Preserve the nullable state of both optional Boolean fields.
• Mark the fields as nullable to match their constructor parameters and getters.
• Add regression coverage for unset values, unrelated request-option merges, and explicit false overrides.

The runtime consumers already use Boolean.TRUE.equals(...), so unset values continue to behave as disabled when requests are created while remaining distinguishable during option merging.

Testing

• ./mvnw -Dmaven.build.cache.enabled=false -pl models/spring-ai-google-genai -am clean package
• 9 reactor modules passed, including 206 tests in spring-ai-google-genai.