Add durable error response parts and turn resume

[roblourens]

Summary

• Represent turn failures as durable ErrorResponsePart entries in the response stream while retaining TurnState.Error as the top-level terminal signal.
• Let hosts mark the final error part with resumable: true; clients decide whether and how to present a resume affordance.
• Add the client-dispatchable chat/turnResume action, which reopens the latest resumable errored turn with the same turn ID, message, response parts, and usage—without adding a synthetic message or turn.
• Preserve prior errors when a resumed turn completes or fails again.

Semantics

The host accepts chat/turnResume only when no turn is active, the requested turn is the latest turn, that turn is errored, and its final response part is resumable. Invalid or stale resume actions are rejected before provider side effects run.

Minimal clients can ignore resumable and continue rendering a coherent terminal error. Clients that support resume can render any appropriate affordance and dispatch chat/turnResume when the user invokes it.

Validation

• Generated documentation, schemas, action-origin metadata, and Swift, Rust, Kotlin, TypeScript, and Go protocol types.
• npm test: 396 passing with 100% reducer branch coverage.
• TypeScript client tests pass.
• Go client tests pass.
• Rust client passes cargo check --tests.

(Written by Copilot)

[Copilot]

Pull request overview

Adds durable turn-error response parts and resumable errored turns across the protocol and generated clients.

Changes:

• Introduces ErrorResponsePart and chat/turnResume.
• Updates reducers and fixtures for resume eligibility and error preservation.
• Regenerates schemas, SDK models, documentation, and changelog metadata.

Show a summary per file

File	Description
types/version/registry.ts	Registers the resume action for protocol 0.8.
types/test-cases/reducers/270-chat-responsepart-cannot-append-error.json	Tests rejecting generic error-part appends.
types/test-cases/reducers/269-chat-turn-resume-completes-one-turn.json	Tests completing a resumed turn.
types/test-cases/reducers/268-chat-turn-resume-preserves-errors-across-retries.json	Tests preserving repeated errors.
types/test-cases/reducers/267-chat-turn-resume-noop-for-unresumable-error.json	Tests unresumable-error rejection.
types/test-cases/reducers/266-chat-turn-resume-noop-for-nonlatest-turn.json	Tests stale-turn rejection.
types/test-cases/reducers/265-chat-turn-resume-noop-for-unknown-turn.json	Tests unknown-turn rejection.
types/test-cases/reducers/264-chat-turn-resume-noop-with-active-turn.json	Tests active-turn rejection.
types/test-cases/reducers/263-chat-turn-resume-reopens-turn.json	Tests reopening an errored turn.
types/test-cases/reducers/250-turncomplete-force-cancels-auth-required-tool-call.json	Updates completed-turn expectations.
types/test-cases/reducers/240-turn-duration-is-clamped-to-zero.json	Updates completed-turn expectations.
types/test-cases/reducers/161-chat-turn-lifecycle-on-chat.json	Updates lifecycle expectations.
types/test-cases/reducers/107-session-input-turn-end-cleans-turn-scoped-only.json	Updates finalized-turn expectations.
types/test-cases/reducers/099-endturn-force-cancels-running-tool-call.json	Updates completed-turn expectations.
types/test-cases/reducers/070-full-turn-flow-with-tool-calls-and-re-confirmation.json	Updates full-flow expectations.
types/test-cases/reducers/017-turncomplete-force-cancels-in-progress-tool-calls.json	Updates completed-turn expectations.
types/test-cases/reducers/016-session-error-finalizes-turn-with-error.json	Moves errors into response parts.
types/test-cases/reducers/015-session-turncancelled-finalizes-turn.json	Removes obsolete null error state.
types/test-cases/reducers/014-session-turncomplete-finalizes-turn.json	Removes obsolete null error state.
types/common/actions.ts	Adds the resume action to common unions.
types/channels-chat/state.ts	Defines durable error response parts.
types/channels-chat/reducer.ts	Implements error finalization and resume behavior.
types/channels-chat/actions.ts	Defines error and resume action contracts.
types/action-origin.generated.ts	Marks resume as client-dispatchable.
scripts/generate-swift.ts	Adds Swift generation metadata.
scripts/generate-rust.ts	Adds Rust generation metadata.
scripts/generate-kotlin.ts	Adds Kotlin generation metadata.
scripts/generate-go.ts	Adds Go generation metadata.
schema/state.schema.json	Adds error-part state schema.
schema/notifications.schema.json	Propagates error-part definitions.
schema/errors.schema.json	Propagates action and error schemas.
schema/commands.schema.json	Propagates action and response schemas.
schema/actions.schema.json	Defines updated chat action schemas.
docs/specification/chat-channel.md	Documents recovery semantics and validation.
docs/guide/state-model.md	Documents durable errors in state.
docs/guide/actions.md	Lists the new resume action.
docs/.changes/20260811-error-response-parts-and-turn-resume.json	Records the user-visible change.
clients/swift/AgentHostProtocol/Tests/AgentHostProtocolTests/ReducersTests.swift	Tests resume dispatchability.
clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/ToolCallStateExtensions.swift	Handles the new response variant.
clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift	Implements Swift reducer behavior.
clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/State.generated.swift	Regenerates Swift state models.
clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/Actions.generated.swift	Regenerates Swift actions.
clients/rust/crates/ahp/src/reducers.rs	Implements Rust reducer behavior.
clients/rust/crates/ahp-types/src/state.rs	Regenerates Rust state models.
clients/rust/crates/ahp-types/src/actions.rs	Regenerates Rust actions.
clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt	Implements Kotlin reducer behavior.
clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/State.generated.kt	Regenerates Kotlin state models.
clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/Actions.generated.kt	Regenerates Kotlin actions.
clients/go/ahptypes/state.generated.go	Regenerates Go state models.
clients/go/ahptypes/actions.generated.go	Regenerates Go actions.
clients/go/ahp/reducers.go	Implements Go reducer behavior.

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• Files reviewed: 47/51 changed files
• Comments generated: 5
• Review effort level: Balanced