[YuqiGuo105]

Link to Issue or Description of Change

Closes #611.

Problem:

POJO properties used by FunctionTool rely on Jackson annotations for their model-facing names and required flags. Applications cannot independently describe tool properties with ADK's @Schema annotation without changing their ordinary business JSON representation. Renaming only the generated schema would also leave tool argument deserialization using the old names.

Solution:

• Allow Annotations.Schema on fields and record components, in addition to methods and parameters.
• Create an isolated copy of the configured ObjectMapper with a schema-aware annotation introspector. Non-empty @Schema.name takes precedence over Jackson's name; @Schema.optional overrides the schema's required marker. Without an ADK annotation, existing Jackson behavior remains in effect.
• Include property descriptions in recursively generated schemas, including nested POJOs and collections.
• Use the same tool-specific mapper for declaration generation, argument binding, and result conversion, keeping the model-facing contract and runtime behavior consistent without mutating the application's mapper.

For example, a property annotated with @JsonProperty("business_id") and @Schema(name = "customer_id", description = "Customer identifier") remains business_id in ordinary business JSON, while the tool schema and tool argument binding use customer_id.

Testing Plan

Unit Tests:

• Added 15 regression tests covering annotation precedence, descriptions, optionality, fields/getters/setters, private fields, record components, nested collections, @JsonIgnore, unannotated POJOs, custom naming strategies, mapper isolation, and return-value/schema consistency.
• ./mvnw -o -B -pl core -Dtest=FunctionToolSchemaTest test passes.
• ./mvnw -B -Prelease -pl core -am clean package passes on OpenJDK 25.0.3 (Java release target 17), including source/Javadoc jars. Both full-core executions (default-test and basic) run 2,904 tests with 0 failures, 0 errors, and 30 skipped; the additional environment-specific executions also pass.
• Google Java formatting produces no changes, and git diff --check upstream/main...HEAD passes.

Manual End-to-End (E2E) Tests:

Not run against a live model. Regression tests exercise local FunctionTool declaration generation and invocation; no model credentials or external services are required.

Checklist

• I have read CONTRIBUTING.md.
• My pull request contains a single commit.
• I have performed a self-review of my own code.
• Added concise Javadocs for annotation behavior.
• Added tests for the feature and backward-compatible Jackson fallback.
• New and existing core unit tests pass locally.
• I have manually tested my changes against a live model.

Additional context

The optional flag controls the generated tool schema's required list; this change does not introduce a separate business-validation framework. This is the Java/Jackson implementation of the requested ADK annotation semantics, rather than a new model-provider protocol.