match lex errors with instanceof, drop getErrorName

`getErrorName` erased the typed error classes by flattening everything to a
string code. Anchor on the classes instead: `LexAuthFactorError` for the login
2FA branch, `XrpcResponseError` for the signup error codes.

`@atproto/lex-client` stays a direct dependency because `XrpcResponseError` is
used at runtime for `instanceof`. It is already in the runtime graph via
`@atproto/lex-password-session`, so the direct declaration adds no bundle
weight; it makes the import legal under pnpm's strict layout and pins the
version so both packages share one `LexError` identity.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/lib/__tests__/lex-error-test.ts

@@ -1,54 +0,0 @@
-import {XrpcResponseError} from '@atproto/lex-client'
-import {LexAuthFactorError} from '@atproto/lex-password-session'
-import {describe, expect, it} from '@jest/globals'
-
-import {getErrorName} from '../lex-error'
-
-/**
- * A stand-in for the method schema an `XrpcResponseError` is built against.
- * The generated lexicons are not available yet, and `XrpcResponseError` only
- * reads `method` back out for `matchesSchemaErrors()`, which these tests never
- * call - so a minimal object is enough.
- */
-const method = {
-  nsid: 'com.atproto.server.createSession',
-  type: 'procedure',
-  errors: ['AuthFactorTokenRequired'],
-} as unknown as ConstructorParameters<typeof XrpcResponseError>[0]
-
-/** An error as the lex client builds one from a JSON error response body. */
-function xrpcResponseError(error: string, message: string) {
-  return new XrpcResponseError(method, new Response(null, {status: 400}), {
-    encoding: 'application/json',
-    body: {error, message},
-  })
-}
-
-describe('getErrorName', () => {
-  it('returns the lexicon error code of an XRPC response error', () => {
-    const e = xrpcResponseError('InvalidToken', 'Bad token scope')
-    expect(getErrorName(e)).toBe('InvalidToken')
-  })
-
-  it('returns AuthFactorTokenRequired for a LexAuthFactorError', () => {
-    /*
-     * The 2FA case: `PasswordSession.login` throws this, and it extends
-     * `LexError` WITHOUT being an `XrpcError` - the reason `getErrorName` is
-     * gated on `LexError`.
-     */
-    const cause = xrpcResponseError(
-      'AuthFactorTokenRequired',
-      'A sign in code has been sent to your email address',
-    )
-    expect(getErrorName(new LexAuthFactorError(cause))).toBe(
-      'AuthFactorTokenRequired',
-    )
-  })
-
-  it('returns undefined for non-lex errors', () => {
-    expect(getErrorName(new Error('InvalidToken'))).toBeUndefined()
-    expect(getErrorName('InvalidToken')).toBeUndefined()
-    expect(getErrorName(null)).toBeUndefined()
-    expect(getErrorName(undefined)).toBeUndefined()
-  })
-})

src/lib/lex-error.ts

@@ -1,13 +0,0 @@
-import {LexError} from '@atproto/lex-client'
-
-/**
- * The lexicon error code (`err.error`). Gated on `LexError` (the base of the
- * lex error hierarchy) rather than `XrpcError` so sibling `LexError` subclasses
- * that are NOT `XrpcError` also surface their `.error` - notably
- * `LexAuthFactorError` (`'AuthFactorTokenRequired'`), which `PasswordSession`
- * throws for email-2FA logins. Every `XrpcError` is a `LexError`, so gating on
- * the base covers server error responses too.
- */
-export function getErrorName(e: unknown): string | undefined {
-  return e instanceof LexError ? e.error : undefined
-}