formatter: make JUnit error testcase names unique

[Kunalbehbud]

Fixes #1608

Problem

#2538 made issue testcase names unique by including the range, but the error testcases appended after them still use the bare diagnostic summary. Multiple diagnostics sharing a summary — e.g. the same Unsupported argument error on two different lines — still produce duplicate <testcase> names inside the same <testsuite>, which JUnit consumers key on.

This is exactly the gap called out in the review of #2522:

junitErrors sets Name: diag.Summary directly, so multiple diagnostics sharing a summary (e.g. two "Unsupported argument" errors on different lines) still produce duplicate <testcase> names in the same <testsuite> (#1608).

and:

this should cover errors too, then LGTM

Change

Include the diagnostic range in the error testcase name, mirroring the issue-testcase naming from #2538 (<summary> <range>, via the existing nil-safe diagRange). The application_error singleton case is untouched.

One edge worth noting: two subject-less diagnostics with an identical summary would still collide (both synthesize the empty range :0,0-0). That mirrors the pre-#2538 state for issues and seems acceptable for a case that can't point at a location — happy to add a positional suffix there if you'd prefer.

Tests

• Updated the existing "diagnostics without subject" golden output for the new name.
• Added a "diagnostics with the same summary" case: two diagnostics sharing a summary on different lines now produce unique <testcase> names.

go test ./formatter/ -run Test_junitPrint

[Kunalbehbud]

The build job failures here are the attestation panic from #2591 — Test_Install_withAttestations crashes on GitHub's new bundle_url responses, unrelated to this formatter change. Proposed a fix in #2593; happy to rebase this branch to re-run CI once that lands.