[EXPERIMENTAL] Delegating SQL execution to FDC Backend Error

### [REQUIRED] Environment info

**firebase-tools:** 15.30.0

**Platform:** Ubuntu

### [REQUIRED] Test case

Deploy Data Connect schema that requires a migration. `schemaValidation: 'COMPATIBLE'` is my setting in `dataconnect.yaml`.

### [REQUIRED] Steps to reproduce

Running `firebase deploy --only dataconnect` with recent versions of `firebase-tools` hits an error if it involves a schema migration:

```
dataconnect: [EXPERIMENTAL] Delegating SQL execution to FDC Backend
```

The error suggests that an experimental feature was enabled, resulting in a failure. I'm not sure if it's specific to my project or how it got turned on.

The error is not present if I revert back to `firebase-tools` version `15.27.0`.

### [REQUIRED] Expected behavior

Schema migration succeeds without errors.

### [REQUIRED] Actual behavior

```
firebase deploy --only dataconnect
i dataconnect: ensuring required API firebasedataconnect.googleapis.com is enabled...
i dataconnect: ensuring required API sqladmin.googleapis.com is enabled...
✔ dataconnect: required API firebasedataconnect.googleapis.com is enabled
✔ dataconnect: required API sqladmin.googleapis.com is enabled
i dataconnect: Generating SQL schema migrations to be compatible...
⚠ dataconnect: PostgreSQL schema is incompatible with the SQL Connect Schema.
Those SQL statements will migrate it to be compatible:

........
........

i dataconnect: [EXPERIMENTAL] Delegating SQL execution to FDC Backend...

Error: Request to https://firebasedataconnect.googleapis.com/v1/projects/abc/locations/northamerica-northeast1/services/abcschemas/main:migrate had HTTP Error: 404, Method not found.
Error: Process completed with exit code 1.
```

Start by reproducing `firebase deploy --only dataconnect` with `schemaValidation: 'COMPATIBLE'` in `dataconnect.yaml`, comparing firebase-tools 15.30.0 with 15.27.0. Trace the Data Connect schema-migration path and the request to the reported `main:migrate` endpoint; done means the migration succeeds without the 404 or experimental delegation error.

Written by the indexing model from the issue text.