Rust: Add Scoped Main-Turn Interruption

[SlowGreek]

Summary

Canonical Rust SDK consumer lane for github/copilot-agent-runtime#12968 and github/github-app#8664:

let result = session
    .interrupt_main_turn(InterruptMainTurnOptions::default())
    .await?;

• Repository-conventional non-exhaustive InterruptMainTurnOptions { flush_queued: bool } with builder API
• Generated typed session.interruptMainTurn RPC and InterruptMainTurnRequest / InterruptMainTurnResult
• SessionCapabilities::interrupt_main_turn: Option<bool> on create/resume snapshots
• Generated typed CapabilitiesChangedData::interrupt_main_turn: Option<bool> event payload
• Complete capabilities.changed snapshots replace the capability cache wholesale by final runtime contract
• No fallback to recursive Session::abort; unsupported or old runtimes surface JSON-RPC MethodNotFound

Parity coverage

Transferred the valuable docs/tests from superseded #2014 while preserving this PR's final-head generated files:

• Exact flushQueued omission through session.rpc().interrupt_main_turn(InterruptMainTurnRequest::default())
• High-level default serializes flushQueued: false; builder serializes true
• interrupted: true and false
• Explicit -32601 propagation with proof that no session.abort follows
• Create true, resume/unsupported false, old omission None
• Full-snapshot replacement plus event.typed_data::<CapabilitiesChangedData>()
• README queue semantics, recursive-abort distinction, and capability-state guidance

Runtime dependency

Generated from the schemas at final runtime head 9391d2e781b23fc1ebcbd5b4605f848afa40b7d0 in github/copilot-agent-runtime#12968. CLI 1.0.71 and current published @github/copilot pins do not contain this protocol. Keep this PR draft until the first published CLI package containing that exact runtime head exists; then update package/version/hash pins through repository conventions and run canonical full codegen. The generated-files freshness check is expected to remain red until that publication gate opens.

Validation

• cargo +nightly-2026-04-14 fmt --check
• cargo clippy --all-features --all-targets -- -D warnings
• cargo test --all-features --test session_test (119 passed)
• GITHUB_ACTIONS=true cargo test --all-features
• RUSTDOCFLAGS='-D warnings' cargo doc --no-deps --all-features

[SlowGreek]

Deduplication note: #2014 is the canonical SDK integration PR and owns ongoing review/maintenance. This draft is temporarily frozen as the final-runtime-head codegen reference. Its generated session_events capability surface and final InterruptMainTurnOptions API will be transferred into #2014 alongside #2014’s broader tests/documentation. Once parity is verified, this PR will close as superseded; no duplicate merge is intended.

[SlowGreek]

Canonical SDK lane. Please preserve this PR’s final-head generated event schema and InterruptMainTurnOptions API. Before readying, transfer from #2014: omitted/false/true wire tests, true/false results, -32601/no-abort-fallback proof, create/resume/old/change capability coverage, typed CapabilitiesChangedData assertion, and README queue/abort/capability documentation. Complete-snapshot replacement on capabilities.changed is intentional per the final runtime contract. #2014 will close after verified parity.

[SlowGreek]

Initial canonical parity transfer is complete at 6b5ad304.

Evidence:

• final-head generated files remain unchanged, including typed CapabilitiesChangedData::interrupt_main_turn: Option<bool> and generated rpc::InterruptMainTurnRequest;
• exact omitted/false/true flushQueued, true/false results, -32601 propagation/no-abort fallback, create/resume/old/change capability states, complete-snapshot replacement, and typed-event decoding are covered;
• README now documents queue semantics, recursive abort distinction, and local/unsupported/old capability values;
• Rust format, Clippy, 119-test session integration suite, complete replay-only Rust suite, and rustdoc pass.

The runtime gate is still closed: github/copilot-agent-runtime#12968 remains open at required head 9391d2e781b23fc1ebcbd5b4605f848afa40b7d0, and no published containing @github/copilot package exists. This PR therefore remains draft and the generated-files freshness check is expected to stay red.

Maintainer action requested after the publication gate opens: the author account has repository viewerPermission: READ, so it cannot enable merge-queue auto-merge. Once canonical pins/codegen are fresh and required checks/review are green, please review and enable repository auto-merge/merge queue. No protection bypass is requested; the hourly steward will keep this lane current.