Add native notification APIs for extension providers and hosts

[fletchto99]

Summary

Add the SDK slice of the coordinated native desktop notification feature:

• Node extensions opt in with joinSession({ requestNotifications: true }) and use session.notifications.getCapabilities(), requestPermission(), and show() from their provider process, including while canvas HTML is hidden.
• All three convenience methods accept non-wire { signal?: AbortSignal } options and forward actual JSON-RPC cancellation. Only capability discovery maps method-not-found to unsupported; delivery is never retried or silently switched to another adapter.
• Add typed Rust NotificationHandler callbacks, ClientOptions::with_notification_handler, explicit host registration status, cancellation-aware dispatch, and unregister-on-stop. Callbacks remain disabled until registration is acknowledged.
• Carry runtime-authenticated origin metadata rather than caller-supplied identity. Clicks are restricted to validated URLs or live canvas-focus receipts; the weak activation context cannot keep its client alive. Sound support is explicit and typed.
• Keep notification payloads out of session events and redact notification errors in SDK transport logs. Add tests and extension/host documentation.

Integration status

Draft: this is not a released, end-to-end feature. Dependent changes in github/copilot-agent-runtime and github/github-app are still in progress. Cross-repository validation and native OS delivery/activation validation are pending. These SDK tests use synthetic JSON-RPC peers; they do not send real notifications or modify real preferences.

The app must use its supported one-way SDK vendoring workflow. The handwritten SDK changes have been reconciled into the in-progress runtime repository's authoritative src/sdk tree; both changes must be included before release export. The runtime enforces trusted stdio-root host nomination and extension authorization; this PR does not grant those permissions itself.

Validation

Passed against the final coordinated runtime schemas:

• npm --prefix nodejs test -- test/extension.test.ts test/notifications.test.ts test/client-global-api.test.ts test/rust-codegen.test.ts — 96 tests passed, rerun after the final schema update.
• npm --prefix nodejs run typecheck — source and test typechecks passed.
• npm --prefix nodejs run build — ESM, CJS, and declaration builds passed.
• Focused ESLint on the changed Node source/tests — passed with six unchanged, pre-existing any-mock warnings in extension.test.ts.
• cargo +1.98.1 test --manifest-path rust/Cargo.toml --no-default-features --features test-support --lib notification — 19 selected tests passed, 310 library tests compiled; two existing dead-code warnings under this minimal feature selection. Rerun after the final schema update.
• cargo +1.98.1 check --manifest-path rust/Cargo.toml --no-default-features --features local-runtime — passed.
• cargo +1.98.1 clippy --manifest-path rust/Cargo.toml --no-default-features --features local-runtime --lib -- -D warnings — passed, including a rerun after the final schema update. The build emits the expected note that the app supplies the local runtime.
• Targeted rustfmt +nightly-2026-04-14 --config-path rust/.rustfmt.nightly.toml --config skip_children=true --check on the six changed handwritten Rust files — passed.
• git diff --cached --check — passed before each commit.

The full-schema test uses COPILOT_RUNTIME_SOURCE=checkout and COPILOT_CLI_SCHEMA_DIR=<runtime-checkout>/generated. Validation was executed across coordinated sessions; final Rust tests and clippy were rerun in this SDK session. Rust toolchains were isolated to session artifacts; no installed app was replaced.

The follow-up commit 30dc82e4 regenerates the final schema-lint metadata. Its only file change is 294 added documentation lines in rust/src/generated/api_types.rs, marking notification variants experimental. No wire fields, values, or handwritten implementation changed.

Schema provenance

Node and Rust projections were generated, not hand-edited, from the coordinated runtime checkout:

• Producer HEAD: github/copilot-agent-runtime at efd3a3f4e9955825a384e9e568433c74a565461c, plus the in-progress notification contract changes.
• Contract-only working diff SHA-256: 0a2fdb0f5923e1d9da71563c77333aa7a272130bc6c33219ce8fd7784074bc77.
• Final api.schema.json SHA-256: f92d2f8f322ebc18fe60b901fa9a800389d5e2c4d27961481fa51af38b8cddc0.
• session-events.schema.json SHA-256: 3d4d826b3a748b902d626ceb82046fff70db2b3bb6449f4c977a38094e275e35.

The runtime reports all 28 schema-lint tests passing for this final schema. The event projection differences are two existing managed-settings documentation updates from the runtime baseline; no notification event or event field was introduced. Published CLI pins and protocol-version constants are unchanged.