Port ExitPlanMode and AutoModeSwitch mode handler APIs from reference implementation

[Copilot]

Resolves #88

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Before the change?

• The Java SDK lacked ExitPlanMode and AutoModeSwitch handler APIs that were restored across other SDKs in reference impl commit 671b50a.
• .lastmerge pointed to 066a69c1, 6 commits behind reference implementation HEAD.

After the change?

• Mode handler types — 8 new files: ExitPlanModeRequest, ExitPlanModeResult, ExitPlanModeInvocation, ExitPlanModeHandler, AutoModeSwitchRequest, AutoModeSwitchResponse (enum: YES/YES_ALWAYS/NO), AutoModeSwitchInvocation, AutoModeSwitchHandler.
• Session wiring — SessionConfig/ResumeSessionConfig gain onExitPlanMode/onAutoModeSwitch fields. SessionRequestBuilder wires handlers to CopilotSession and sets requestExitPlanMode/requestAutoModeSwitch capability flags on Create/ResumeSessionRequest.
• RPC dispatch — RpcHandlerDispatcher registers exitPlanMode.request and autoModeSwitch.request methods. Default behavior: approve exit (no handler) / decline mode switch (no handler).
• Tests — 11 unit tests (clone, request flags, enum serialization), 2 E2E tests (@Disabled pending snapshot availability after .lastmerge update).
• Docs — Mode Handlers section added to advanced.md.
• Sync housekeeping — .lastmerge → ce56eb81, copilot version → ^1.0.44-2.

Usage:

var config = new SessionConfig();
config.setOnExitPlanMode((request, invocation) -> {
    var result = new ExitPlanModeResult();
    result.setApproved(true);
    return CompletableFuture.completedFuture(result);
});
config.setOnAutoModeSwitch((request, invocation) -> {
    return CompletableFuture.completedFuture(AutoModeSwitchResponse.YES);
});

Commits not ported (not applicable to Java SDK): ce56eb8 (typo not present), 299ea21/97f505c (CI-only), b0d1c8e (Rust-only), e759700 (.NET generated code).

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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