feat(core): content block factories + ids + docs + tests (#32316)

## Benefits

1. **Type Safety**: Compile-time validation of required fields and
proper type setting
2. **Less Boilerplate**: No need to manually set the `type` field or
generate IDs
3. **Input Validation**: Runtime validation prevents common errors
(e.g., base64 without MIME type)
4. **Consistent Patterns**: Standardized creation patterns across all
block types
5. **Better Developer Experience**: Cleaner, more intuitive API than
manual TypedDict construction. Also follows similar other patterns (e.g.
`create_react_agent`, `init_chat_model`

.github/workflows/codspeed.yml

@@ -20,6 +20,7 @@ jobs:
   codspeed:
     name: 'Benchmark'
     runs-on: ubuntu-latest
+    if: ${{ !contains(github.event.pull_request.labels.*.name, 'codspeed-ignore') }}
     strategy:
       matrix:
         include: