[SDK] Migrate the post pipeline and blob uploads (#11380)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/lib/api/legacy-blob.ts

@@ -0,0 +1,31 @@
+import {BlobRef} from '@atproto/api'
+import {type BlobRef as LexBlobRef} from '@atproto/lex'
+
+/**
+ * Bridge a lex blob ref (the plain-JSON `{$type: 'blob', ref, mimeType, size}`
+ * that {@link uploadBlob} now returns) back to the legacy `BlobRef` class
+ * instance.
+ *
+ * Only needed where a blob is handed to a legacy agent write: the legacy
+ * lexicon blob validator checks `value instanceof BlobRef`, so a plain lex
+ * blob fails validation, and the legacy serializer would put the wrong shape
+ * on the wire. Drop each call as its write moves to the lex client.
+ */
+export function toLegacyBlobRef(blob: LexBlobRef): BlobRef {
+  return BlobRef.fromJsonRef(blob as Parameters<typeof BlobRef.fromJsonRef>[0])
+}
+
+/**
+ * Normalize a legacy `BlobRef` class instance to the plain-JSON lex blob shape.
+ *
+ * Required for any blob that reaches a lex write: the lex serializer walks
+ * plain objects, so a class instance goes on the wire with its internal
+ * `original` field and no `$type`. `ipld()` yields exactly the lex shape, and
+ * hashes identically (see `computeCid.test.ts` case 2b).
+ *
+ * Only the video pipeline still needs this - it reads its blob off the legacy
+ * agent (`app.bsky.video.getJobStatus`). Drop it when the video client moves.
+ */
+export function fromLegacyBlobRef(blob: BlobRef): LexBlobRef {
+  return blob.ipld()
+}