Add custom agent handoff metadata to Rust SDK

[pierceboggan]

Summary

• Add generated CustomAgentHandoff with required label and agent, plus optional prompt, send, and model fields.
• Expose optional ordered handoffs on generated AgentInfo, covering agents.discover and session agent RPC results.
• Add matching CustomAgentConfig.handoffs support and a with_handoffs builder that reuse the generated handoff type.

Schema and compatibility

The generated contract matches the canonical runtime schema from github/copilot-agent-runtime@396e626c. The CustomAgentHandoff definition and AgentInfo.handoffs property were checked for exact schema equality.

Option<Vec<CustomAgentHandoff>> preserves missing handoff metadata for older runtimes and authored display order when present. Each optional item field remains omitted on serialization when unset. Existing AgentInfo payloads without handoffs continue to deserialize.

Testing

• cargo +nightly-2026-04-14 fmt --check
• COPILOT_SKIP_CLI_DOWNLOAD=1 cargo clippy --all-features --all-targets -- -D warnings
• COPILOT_SKIP_CLI_DOWNLOAD=1 cargo test --all-features --lib (243 passed)
• COPILOT_SKIP_CLI_DOWNLOAD=1 cargo test --all-features --test api_types_test (13 passed)
• COPILOT_SKIP_CLI_DOWNLOAD=1 COPILOT_CLI_PATH=/opt/homebrew/bin/copilot cargo test --all-features --test e2e rpc_agent:: (7 passed)

[Copilot]

Copilot review overview

🟡 Changes recommended

The generated Rust additions are not reproducible from the repository’s pinned schema dependency and would fail the codegen check.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review tier: Balanced
Findings: None

What changed in this PR

Adds custom-agent handoff metadata and configuration support to the Rust SDK.

Changes:

• Adds generated handoff and agent metadata types.
• Adds CustomAgentConfig::with_handoffs.
• Tests serialization, ordering, omission, and RPC exposure.

File	Description
rust/​tests/​api_types_test.rs	Tests handoff serialization and RPC payloads.
rust/​src/​types.rs	Exposes handoffs through custom-agent configuration.
rust/​src/​generated/​api_types.rs	Adds generated handoff protocol types.

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[SteveSandersonMS]

Thanks for the work here. I verified this handoff metadata is not yet present in the runtime schema currently consumed by this repo, and the Rust RPC additions are therefore not codegen-derived from our pinned source.

I’m moving this PR to draft for now. Please mark it ready for review once the corresponding runtime change has landed and this PR references that shipped runtime version, with the RPC type changes coming purely from codegen output.

For tracking, I've moved this to draft - please mark as ready to review when appropriate.