go: plumb context cancellation through ToolInvocation for session.Abort()

[gimenete]

Summary

Closes #1433 (Go SDK portion)

When session.Abort() is called, in-flight tool handlers now receive a cooperative cancellation signal via ToolInvocation.Context. A new session.CancelToolCall(toolCallID) method lets callers cancel a single handler without affecting others or the agentic loop.

Changes

go/types.go

• Added Context context.Context to ToolInvocation — carries W3C Trace Context (OTel) and is cancelled when session.Abort() or session.CancelToolCall() is called.
• Deprecated TraceContext in favour of Context (same value; existing code using TraceContext continues to work unchanged).

go/session.go

• Added toolCallCancels map[string]context.CancelFunc + mutex to Session to track in-flight tool call cancel funcs.
• executeToolAndRespond creates a context.WithCancel child of the trace context and registers its cancel func; cleans up (deregisters + calls cancel()) when the handler returns or panics.
• Two separate defers (LIFO) ensure panic recovery runs before cleanup, so the error RPC response is still sent when a panic occurs.
• RPC response calls (HandlePendingToolCall) use traceCtx (non-cancellable) so they succeed even if Abort() already fired.
• Abort() cancels all stored cancel funcs and removes them from the map after the RPC acknowledgment.
• CancelToolCall(toolCallID string) bool — cancels a single in-flight handler by ID. Returns true if found and cancelled, false if not in flight. Idempotent.

go/session_test.go

• Extract newRPCDrainSession helper (pipe-based mock RPC, drains all requests with empty success responses).
• TestToolInvocation_ContextCancelledOnAbort — handler context cancelled by abort simulation; map cleaned up.
• TestToolInvocation_ContextPopulated — Context and TraceContext both set and equal.
• TestCancelToolCall_cancelsTargetedHandlerOnly — unknown ID returns false; targeted handler cancelled; sibling handler stays live; second call on same ID returns false.

go/README.md

• "Cooperative Cancellation via session.Abort" section with HTTP example.
• "Cancelling a single tool call" section with CancelToolCall example and ToolCallID capture pattern.

Backwards compatibility

Existing handlers that ignore inv.Context / inv.TraceContext are unaffected. TraceContext holds the same value as before (additive change).

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR introduces cooperative cancellation for tool handlers by propagating session.Abort() into a new ToolInvocation.Context, while preserving trace propagation for OpenTelemetry.

Changes:

• Add ToolInvocation.Context (cancelled on Session.Abort) and deprecate direct usage of ToolInvocation.TraceContext.
• Track in-flight tool call cancel functions in Session and cancel them during Abort.
• Add tests and documentation describing cancellation behavior for tool handlers.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
go/types.go	Adds ToolInvocation.Context and deprecation guidance for TraceContext.
go/session.go	Implements per-tool-call cancellation tracking and abort-triggered cancellation; uses a non-cancelled context for RPC responses.
go/session_test.go	Adds tests validating handler context cancellation on abort and context population.
go/README.md	Documents cooperative cancellation via ToolInvocation.Context.