java: add schema attribute to @CopilotToolParam for custom type schema override

[rinceyuan]

Closes #1794

Summary

Adds an optional schema attribute to @CopilotToolParam that allows users to explicitly specify the JSON Schema for a tool parameter. This enables support for custom/third-party types that the SchemaGenerator cannot automatically map.

Example

@CopilotTool("Schedule a deployment")
public String scheduleDeployment(
        @CopilotToolParam(value = "Deployment time in ISO-8601",
            schema = "{\"type\":\"string\",\"format\":\"date-time\"}")
        com.acme.internal.AcmeDateTime deployTime) {
    return "Scheduled for " + deployTime;
}

Changes

CopilotToolParam.java

• Added String schema() default "" annotation attribute with Javadoc

CopilotToolProcessor.java

• In generateSchemaWithParamMetadata(): when schema is non-empty, use it instead of auto-generated schema
• Added compile-time validation: schema + defaultValue conflict emits error
• Added compile-time validation: invalid JSON (not starting/ending with braces) emits error
• Added jsonToMapOfSource() — a minimal recursive-descent JSON parser that converts JSON strings to Map.of(...) source code expressions at compile time (no external JSON library needed)

Param.java

• Added schema field, fluent schema() mutator, and getter for lambda API parity
• Updated equals(), hashCode() to include schema

CopilotToolProcessorTest.java (4 new tests)

1. Schema override — custom schema appears in generated code
2. Schema + defaultValue conflict — produces compile error
3. Invalid JSON — produces compile error
4. Empty schema — falls through to normal type-based generation

Testing

• All 4 new tests pass
• The only failing test (generatesNullMetadata_whenAbsent) is a pre-existing failure on main unrelated to this PR

[rinceyuan]

@microsoft-github-policy-service agree company=Microsoft

[rinceyuan]

@edburns Hi! This implements #1794 (sibling of #1836) — adds schema attribute to @CopilotToolParam for custom type schema overrides. 4 files changed, 4 new tests. Would appreciate your review. Thanks!