Compute `NotebookIndex` for `Diagnostics` on stdin (#7663)
## Summary
This PR fixes the bug where the `NotebookIndex` was not being computed
when
using stdin as the input source.
## Test Plan
On `main`, the diagnostic output won't include the cell number when
using stdin
while it'll be included after this fix.
### `main`
```console
$ cat ~/playground/ruff/notebooks/test.ipynb | cargo run --bin ruff -- check --isolated --no-cache - --stdin-filename ~/playground/ruff/notebooks/test.ipynb
/Users/dhruv/playground/ruff/notebooks/test.ipynb:2:8: F401 [*] `math` imported but unused
/Users/dhruv/playground/ruff/notebooks/test.ipynb:7:8: F811 Redefinition of unused `random` from line 1
/Users/dhruv/playground/ruff/notebooks/test.ipynb:8:8: F401 [*] `pprint` imported but unused
/Users/dhruv/playground/ruff/notebooks/test.ipynb:12:4: F632 [*] Use `==` to compare constant literals
/Users/dhruv/playground/ruff/notebooks/test.ipynb:13:38: F632 [*] Use `==` to compare constant literals
Found 5 errors.
[*] 4 potentially fixable with the --fix option.
```
### `dhruv/notebook-index-stdin`
```console
$ cat ~/playground/ruff/notebooks/test.ipynb | cargo run --bin ruff -- check --isolated --no-cache - --stdin-filename ~/playground/ruff/notebooks/test.ipynb
/Users/dhruv/playground/ruff/notebooks/test.ipynb:cell 3:2:8: F401 [*] `math` imported but unused
/Users/dhruv/playground/ruff/notebooks/test.ipynb:cell 5:1:8: F811 Redefinition of unused `random` from line 1
/Users/dhruv/playground/ruff/notebooks/test.ipynb:cell 5:2:8: F401 [*] `pprint` imported but unused
/Users/dhruv/playground/ruff/notebooks/test.ipynb:cell 6:2:4: F632 [*] Use `==` to compare constant literals
/Users/dhruv/playground/ruff/notebooks/test.ipynb:cell 6:3:38: F632 [*] Use `==` to compare constant literals
Found 5 errors.
[*] 4 potentially fixable with the --fix option.
```
