Use SourceKind::diff for formatter

[dhruvmanila]

Summary

This PR refactors the formatter diff code to reuse the SourceKind::diff logic. This has the benefit that the Notebook diff now includes the cell numbers which was not present before.

Test Plan

Update the snapshots and verified the cell numbers.

[dhruvmanila]

We could potentially create a new SourceError::IncompatibleDiffSource variant but then the Diagnostics:from_source_error needs to be updated which I don't think is correct because the diff error shouldn't really be a diagnostic.

[charliermarsh]

I really wish we had a way to enforce this statically through types (i.e., that this path should never be reached).

[github-actions]

PR Check Results

Ecosystem

ℹ️ ecosystem check detected changes. (+0, -0, 1 error(s))

build (error)

https://github.com/pypa/build ref main select ignore exclude

ruff failed
  Cause: TOML parse error at line 2, column 19
  |
2 | requires = ['bad' 'syntax']
  |                   ^
invalid array
expected `]`

[konstin]

thanks!