Unable to save slice for table created from Semantic View source

### Bug description

1. Configure a semantic view connection ( i created a custom source as none exist in Superset)
2. Add a Data Source
3. Goto Charts -> New. Select new semantic view source -> Table visualisation
4. Configure dimensions and metrics -> Create (successful to show the data table)
5. Save (Failure to save slice)

`chart/` endpoint returns the following payload:

`{"message":{"datasource_type":["Datasource type is invalid"]}}`

I reviewed the following issue and was able to resolve this myself:
- https://github.com/apache/superset/pull/43500
- https://github.com/apache/superset/issues/43552

### Screenshots/recordings

Chart renders before save:

Error screenshot:

### Superset version

- Superset base image: apache/superset@sha256:7dd2472be816c7bf419d0f5088d58940f4e1c0b7ba16bf8defa453162f496d84
- Superset Git ref: fbd75f00865be08905975672ddd622b8ab687989
- Installed Superset package version: 0.0.0.dev0

### Python version

3.12.14

### Node version

24.21.0

### Browser

Chrome

### Additional context

Since my connector is custom and renders the data as a table or other visualisation, I presume it is not a connector issue for semantic views and more of a Superset chart saving issue. However I'm happy to provide any details on the custom connector privately, if required.

Only the following log in the web container:
`2026-09-11T09:01:26.3436570Z stdout F [IP addr redacted] POST /api/v1/chart/ 422 0.132461`

### Checklist

- [x] I have searched Superset docs and Slack and didn't find a solution to my problem.
- [x] I have searched the GitHub issue tracker and didn't find a similar bug report.
- [ ] I have checked Superset's logs for errors and if I found a relevant Python stacktrace, I included it here as text in the "additional context" section.

Start at the POST /api/v1/chart/ endpoint and compare its semantic view save path with the changes discussed in PR #43500 and issue #43552. Reproduce the table visualization flow, then verify that saving a chart from a semantic view source succeeds without the “Datasource type is invalid” response.

Written by the indexing model from the issue text.