[BUG] [VSCE] [LSP] Source auto complete doesn't work

**Describe the bug**
A clear and concise description of what the bug is.

**What version of dbt Fusion is this bug in? (find out by running `dbt --version`)**
```
=== dbt System Report - System Info ===

Timestamp: 2026-05-07T18:24:38.248Z
Cookie ID: ed472d67-18ad-4621-abad-debb5f16772b

--- Editor ---
Editor: visual studio code
Editor version: 1.119.0

--- Extension ---
Extension version: 0.68.0
Expected LSP ver: 2.0.0-preview.175
dbt binary path: /Users/jeremy/.local/bin/dbt
dbt version: dbt-fusion 2.0.0-preview.175

--- LSP Instances ---
/Users/jeremy/git/dbt-basic
status: running
version: 2.0.0-preview.175

--- System ---
Platform: darwin
OS release: 24.6.0
Architecture: arm64
Node version: v22.22.1
```

**Is this a discrepancy between the dbt Fusion Engine and dbt Core? Check one.**
- [ ] YES
- [ ] NO

**To Reproduce**
```yaml
# dbt_project.yml
name: dbt_basic_5
profile: sf
version: "1.0.0"

models:
dbt_basic_5:
+materialized: table

# models/sources.yml
sources:
- name: raw
database: db
tables:
- name: customers
```

```sql
-- models/foo.sql
select 1 c
```

Add a new model `bar.sql` then try to autocomplete with `ref()` / `source()`):

^ After saving the new model file, we see `ref()` autocomplete work but not `source()` autocomplete.

**Expected behavior**
`source()` autocomplete should work too :)

**Operating System and CPU Type (please complete the following information):**
* Mac/Windows/Linux? Mac
* X86 or ARM? ARM

Reported internally here: https://dbt-labs.slack.com/archives/C08PYQS8B7Z/p1778175994303209 - will share above repro generated vsce system report in that thread too.