Add private app extension APIs

[merlynomsft]

Summary

• Add the private @github/copilot-sdk/private/app-extension entry point and defineAppExtension(async host => disposer) activation model.
• Add typed Node and Rust identities, capability grants, contribution declarations, lifecycle cancellation, and generated private handshake contracts.
• Expose frozen, capability-limited hosts for session presentations, app canvases, forge providers, and mediated fetch without exposing the backing CopilotSession, CopilotClient, raw JSON-RPC, credentials, generic session mutation, or unrestricted fetch.
• Add atomic Create PR presentation/action support and optional requiredTool routing for product-session prompts.
• Keep joinAppSessionBadges() and legacy setBadge/setBadges APIs exported as compatibility adapters; legacy badge updates preserve the current action.

Private API shape

• host.sessionBadges.register({ contributionId, onAction? }) receives revisioned target snapshots and publishes singular or atomic ordered presentation updates.
• AppSessionPresentation combines badge with an optional Create PR action state; setPresentation and setPresentations replace both atomically.
• onAction receives the exact eligible snapshot target, kind: "createPullRequest", the requested draft mode, and an AbortSignal; it returns { prompt, requiredTool } or null.
• host.canvases.register(...) supports bounded generic action descriptors and typed open/action/close callbacks for a declared contribution.
• host.forgeProviders.register(...) exposes named provider operations for a declared contribution.
• host.mediatedFetch.request(...) requests a host-authorized operation without receiving account credentials.
• session.send({ prompt, requiredTool? }) forwards an optional required tool in Node and Rust.

Lifecycle and security boundaries

• extensions.appExtension.register({ protocolVersion: 1 }) returns runtime-derived principal, capability grants, and declared contribution identities; the SDK sends no spoofable identity.
• Registrations are contribution-scoped, race-safe, and cleaned up exactly once on explicit disposal, transport closure, or cancellation.
• Action prompts are limited to 32 KiB and must begin with the exact # Pull Request Creation header; required tool identifiers are strictly validated.
• The SDK exposes no generic app-host session send API. Runtime validates that requiredTool belongs to an active same-package product-session extension.
• Presentation lifecycle resets are represented by an authenticated event with presentation: null.

Dependencies and landing order

1. This PR — SDK contracts and private host API.
2. github/copilot-agent-runtime#19512 — trusted private bootstrap and transport.
3. github/github-app#14999 — app host and contribution consumers.

Validation

• Node typecheck and build.
• 45 focused Node unit/codegen/child-stdio transport tests.
• ESLint on all changed Node source and test files.
• Rust Clippy with all features and targets and warnings denied.
• Focused Rust app-extension, presentation-event, and requiredTool transport tests.
• Deterministic TypeScript/Rust codegen reruns.
• The full Rust non-E2E suites passed. Full Rust E2E startup was blocked by the configured package registry lacking the locked fast-uri@3.1.7 harness dependency; affected transport coverage is green.

[github-actions]

SDK Consistency Review — PR #2574

No cross-SDK consistency issues found.

This PR adds a private/internal app-extension API surface:

• @github/copilot-sdk/private/app-extension entry point, defineAppExtension(), joinAppSessionBadges(), canvas/forge-provider registration, and mediated fetch.
• Implemented consistently in both Node.js and Rust (new files: nodejs/src/appExtension.ts, nodejs/src/appSessionBadges.ts, nodejs/src/extensionSession.ts; rust/src/app_extension.rs, rust/src/app_session_badges.rs), plus shared codegen (scripts/codegen/app-extension-api.schema.json, rust.ts, typescript.ts).

Why this doesn't need Python/Go/.NET/Java parity right now:

• The wire schema explicitly marks all new types "visibility": "internal", and the Node/Rust code is annotated @internal / #[doc(hidden)].
• The PR description states this is a private handshake consumed only by GitHub's own app host (github/github-app) and CLI runtime (github/copilot-agent-runtime), not a general public feature for third-party SDK consumers.
• The two languages implemented (Node.js, Rust) match the languages actually required by the dependent integration PRs listed in the "Dependencies and landing order" section.

API naming/shape is parallel between the two implemented languages (joinAppSessionBadges/AppSessionBadgesExtension in TS vs. app_session_badges/equivalent types in Rust), following each language's idioms. No action needed for the remaining SDKs unless/until this private surface is promoted to a public, cross-language feature.

Generated by SDK Consistency Review Agent for #2574 · copilot · sonnet50 · 40.8 AIC · ⌖ 12.3 AIC · ⊞ 8.3K · ◷