[TylerLeonhardt]

Summary

Follow up on #583 with an optional AuthenticateParams.accountLabel, covering the display label currently carried in private metadata by microsoft/vscode#347244.

• Define the label as display-only: it is not an account identifier or authorization input and need not be unique or stable.
• Keep it optional independently of accountId, with no capability or protocol-version change.
• Regenerate native client models and JSON schemas, update the authentication documentation and changelog fragment, and add schema validation plus a shared round-trip fixture.

Wire compatibility is preserved. Rust struct literals must add account_label: None when omitting the label; existing Kotlin data-class binaries must be rebuilt.

Validation

• npm run generate
• npm run test — 672 passed, including typecheck, lint, generation checks, schema tests, and reducer coverage.
• Protocol and TypeScript client typechecks passed again before opening this draft.
• Shared round-trip corpus passed for TypeScript, Rust, Go, Kotlin, and .NET (.NET: 115 tests).
• Swift's generated AuthenticateParams passed the three shared authentication round-trip fixtures through a temporary SwiftPM executable. The full XCTest suite could not run because the local command-line toolchain lacks XCTest.
• git diff --check