Add open-canvases snapshot tracking to the Java SDK

[jmoseley]

What

Brings the Java SDK to parity with the other five SDK languages (Rust, Node, Python, Go, .NET) by maintaining an in-memory snapshot of the canvas instances currently open for a session. Java previously had none of this feature — only the generated event/data types existed — so this is net-new.

Behavior (mirrors the other 5 languages' contract exactly)

• CopilotSession keeps a lock-guarded List<OpenCanvasInstance> and exposes it via a new public accessor getOpenCanvases() (returns an immutable defensive copy).
• session.canvas.opened → upsert by instanceId. A provider-unregister re-emit arrives as another opened event with availability=stale and replaces the prior entry rather than duplicating it.
• session.canvas.closed → remove by instanceId (idempotent — removing an absent id is a no-op).
• Validation guardrails: opened requires non-empty instanceId/canvasId/extensionId and non-null availability; closed requires a non-empty instanceId. Invalid/empty/null payloads log the canonical failed to deserialize session.canvas.{opened,closed} payload warning and no-op. opened/stale events are never treated as removals.
• The update runs inside handleBroadcastEventAsync alongside the existing capabilities.changed passive state update, before user handlers observe the event, wrapped best-effort so snapshot upkeep can never disrupt event delivery.
• Resume/create seeding: the snapshot is seeded from the session.create / session.resume responses, which already carry openCanvases on the wire. Added the field to the hand-written CreateSessionResponse / ResumeSessionResponse records and seed via session.setOpenCanvases(...) after setCapabilities, matching .NET.

Tests

New SessionCanvasSnapshotTest (12 cases): open two → both present; close one → gone, other remains; idempotent absent close; empty + null instanceId no-op; missing-required-field opened ignored; stale re-emit replaces (no duplicate); getOpenCanvases() returns an immutable copy; seed via setOpenCanvases (incl. null-element filtering); null clears; plus Jackson deserialization tests for both response records. mvn spotless:apply && mvn verify is green.

Context

• Runtime events: copilot-agent-runtime #9489, shipped in CLI 1.0.60.
• Mirrors Handle session.canvas.closed by removing from open_canvases snapshot #1604, which landed the same opened-upsert + closed-remove behavior for Rust/Node/Python/Go/.NET (Java was deferred because it had no snapshot feature at all).
• Consumer: github-app's sticky-canvas fix.

Constraints honored

Java only. No edits to generated files (java/src/generated/) or other languages.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[Copilot]

Pull request overview

Adds Java-side in-memory tracking of “open canvas instances” for a session, bringing the Java SDK in line with the existing snapshot behavior in the other language SDKs.

Changes:

• Added a lock-guarded open-canvases snapshot to CopilotSession, updated via session.canvas.opened (upsert) and session.canvas.closed (remove), and exposed via getOpenCanvases().
• Seeded the snapshot from session.create / session.resume RPC responses by adding openCanvases to the corresponding response records and wiring seeding in CopilotClient.
• Added unit tests covering upsert/remove, guardrails, stale re-emit replacement, seeding, and response deserialization.

Show a summary per file

File	Description
java/src/main/java/com/github/copilot/CopilotSession.java	Maintains and exposes the open-canvases snapshot; updates it during event dispatch.
java/src/main/java/com/github/copilot/CopilotClient.java	Seeds the session’s open-canvases snapshot from create/resume responses.
java/src/main/java/com/github/copilot/rpc/CreateSessionResponse.java	Adds openCanvases to the create-session response shape for seeding.
java/src/main/java/com/github/copilot/rpc/ResumeSessionResponse.java	Adds openCanvases to the resume-session response shape for seeding.
java/src/test/java/com/github/copilot/SessionCanvasSnapshotTest.java	New unit coverage for snapshot behavior + response deserialization.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 4

[github-actions]

Cross-SDK Consistency Review ✅

This PR correctly brings the Java SDK to full parity with the other five SDK implementations. I reviewed the implementation against the current main branch (which already includes PR #1604's changes for Rust/Node/Python/Go/.NET).

Feature parity across all SDKs

SDK	Public accessor	opened → upsert	closed → remove	Seeded on create/resume	Thread-safe
Node.js	openCanvases (getter)	✅	✅	✅	✅
Python	open_canvases (property)	✅	✅	✅	✅
Go	OpenCanvases()	✅	✅	✅	✅
.NET	OpenCanvases (property)	✅	✅	✅	✅
Rust	open_canvases()	✅	✅	✅	✅
Java (this PR)	getOpenCanvases()	✅	✅	✅	✅

API naming consistency

Java uses getOpenCanvases() — idiomatic Java getter convention — which correctly mirrors the property/accessor pattern used in all other languages. Validation guardrails (non-empty instanceId/canvasId/extensionId, non-null availability for opened; non-empty instanceId for closed) match the other SDKs exactly. The method returns an immutable defensive copy (List.copyOf), consistent with Node.js ([...spread]), Go (copy), Python (list(...)), and .NET (AsReadOnly()).

No inconsistencies found. 🎉

Generated by SDK Consistency Review Agent for issue #1606 · sonnet46 3M · ◷