Port mode handler APIs (ExitPlanMode, AutoModeSwitch) from reference implementation

[Copilot]

Resolves #196

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Before the change?

• Java SDK lacked ExitPlanMode and AutoModeSwitch handler APIs present in other SDK implementations
• ModelBilling.multiplier was a primitive double, couldn't represent absent values
• .lastmerge pointed to 066a69c, @github/copilot at prior version

After the change?

• Mode handler APIs — Ports 671b50a (Restore mode handler APIs across SDKs):
  • ExitPlanModeHandler / AutoModeSwitchHandler functional interfaces with request/result/invocation types
  • SessionConfig and ResumeSessionConfig gain onExitPlanMode / onAutoModeSwitch fields (with clone support)
  • CreateSessionRequest / ResumeSessionRequest gain requestExitPlanMode / requestAutoModeSwitch capability flags
  • SessionRequestBuilder wires handlers and flags automatically
  • CopilotSession registers handlers via AtomicReference, dispatches with safe defaults (approved=true / NO)
  • RpcHandlerDispatcher handles exitPlanMode.request and autoModeSwitch.request RPC methods
• Nullable multiplier — ModelBilling.multiplier changed from double to Double (from e759700)
• Version sync — .lastmerge → f625779, @github/copilot → ^1.0.44-3

Not ported (not applicable to Java): Go/Rust/Node/Python SDK changes, CI fork restrictions, generated code changes, doc typos in ref-impl-only files.

var config = new SessionConfig()
    .setOnExitPlanMode((request, invocation) -> {
        // Review plan before exiting plan mode
        return CompletableFuture.completedFuture(
            new ExitPlanModeResult().setApproved(true));
    })
    .setOnAutoModeSwitch((request, invocation) -> {
        // Handle rate-limit mode switch prompt
        return CompletableFuture.completedFuture(AutoModeSwitchResponse.YES);
    });

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

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