Add junit-xml output format

[nitishagar]

Summary

• Adds OutputFormat::JunitXml (--output-format junit-xml, or output-format = "junit-xml" in pyrefly.toml) emitting a JUnit-XML <testsuites> document.
• Mirrors the existing Json / Github writer pattern in pyrefly/lib/commands/check.rs. No new dependencies — hand-rolled XML escaping (attributes and CDATA, including ]]> splitting).
• One <testcase> per error, with classname carrying the file path, name carrying error_kind:Lline, plus file and line attributes for CI dashboards that bucket per file.
• Severity mapping: error and warn produce <failure> entries (warnings rendered as type="warning"); info and ignore are dropped, matching the existing JSON writer.

Fixes #3389

Test plan

• cargo test -p pyrefly --lib — 5157/5157 passing locally (includes the two new tests for structure and escaping).
• cargo test -p pyrefly_config test_output_format_config_parsing — 1/1.
• New scrut block in test/errors.md covering --output-format junit-xml.
• CI green.

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[samwgoldman]

Hey, can you rebase this change? I'm having trouble importing it because of the merge conflict in Cargo.lock. Thanks!

[nitishagar]

@samwgoldman Rebased onto latest main and resolved the Cargo.lock conflict, should be importable now. Thanks!

[meta-codesync]

@samwgoldman has imported this pull request. If you are a Meta employee, you can view this in D105751390.