refactor: use Optional return types instead of nullable boxed primitives in public API

[Copilot]

Resolves #187

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Before the change?

• ~30 public getters on mutable config/builder classes return nullable boxed types (Boolean, Integer, Double) — a C# idiom carried over from the reference implementation. The "may be absent" contract is invisible in type signatures and forgotten null-checks compile fine but NPE at runtime.

After the change?

• Getters return Optional<Boolean>, OptionalInt, or OptionalDouble. Setters take primitives (boolean, int, double). New clearXxx() methods reset to null (server default).
• Internal fields remain nullable boxed types — Jackson serialization is unchanged. @JsonIgnore on Optional-returning getters prevents Jackson from attempting to serialize Optional wrappers.
• All callers updated from null-check patterns to Optional APIs:

// Before
request.setEnableSessionTelemetry(config.getEnableSessionTelemetry());

// After
config.getEnableSessionTelemetry().ifPresent(request::setEnableSessionTelemetry);

Affected classes (15):

• Public config: CopilotClientOptions, SessionConfig, ResumeSessionConfig, InfiniteSessionConfig, InputOptions, ModelCapabilitiesOverride.Supports, ModelCapabilitiesOverride.Limits, ProviderConfig, TelemetryConfig, SessionUiCapabilities, CustomAgentConfig, UserInputRequest
• Wire DTOs: CreateSessionRequest, ResumeSessionRequest, McpServerConfig, ModelLimits

Updated callers: CliServerManager, CopilotClient, CopilotSession, SessionRequestBuilder

Not changed: Records, generated code under src/generated/java/, primitive fields that were already non-nullable.

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• mvn spotless:apply has been run to format the code
• mvn clean verify passes locally

Does this introduce a breaking change?

• Yes
• No

Getter return types change from Boolean/Integer/Double → Optional<Boolean>/OptionalInt/OptionalDouble. Setter parameter types change from boxed → primitive. Callers passing null to setters must use the new clearXxx() methods instead.

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