Merge reference implementation SDK changes (2026-05-17)

[Copilot]

Ports 9 commits from github/copilot-sdk (e20f5bef→25b15be8).

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Before the change?

• CustomAgentConfig had no way to specify a preferred model for an agent
• Permission-reject E2E test only verified the file wasn't modified, not that the CLI surfaced the correct "user rejected" error discriminator (copilot-sdk#1194)
• @github/copilot pinned at previous version

After the change?

• CustomAgentConfig.setModel(String) — optional model identifier (e.g. "claude-haiku-4.5"); runtime falls back to parent session model if unavailable
• testDenyPermission now asserts a ToolExecutionCompleteEvent with "user rejected" error, matching the hardened .NET test
• @github/copilot synced to ^1.0.49-1, .lastmerge updated

Not ported (intentionally)

Commit	Reason
25b15be .NET MSBuild targets	Not applicable to Java/Maven
31ea472 / 59f0981 Python doc fixes	Python-only
bc0f1c4 ask_user snapshot consolidation	Java SDK already uses canonical ask_user folder
fc11032 Compaction E2E stabilization	Test harness (Node.js) only
Session.cs reasoningSummary param	Requires codegen regeneration from updated @github/copilot types

Example

var agent = new CustomAgentConfig()
    .setName("code-reviewer")
    .setModel("claude-haiku-4.5")
    .setPrompt("You are a code review expert.");

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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