finish phase 4: drop @atproto/api dependency, lex-only errors, delete toLex and legacy validators

oxlint-suppressions.json gains a baseline entry for webpack.config.js: the
file was never previously staged/linted (lint script covers src+modules
only), and editing it exposed its pre-existing violations to lint-staged.
All suppressed counts are pre-existing; this change only removes a line.

__tests__/lib/string.test.ts

@@ -1,4 +1,4 @@
-import {RichText} from '@atproto/api'
+import {RichText} from '@bsky.app/sdk/richtext'
 import {i18n} from '@lingui/core'
 
 import {parseEmbedPlayerFromUrl} from '#/lib/strings/embed-player'