fix: handle undefined code field in oxlint diagnostics

[skoshx]

Root Cause

The error occurs in parseRuleCode when processing oxlint diagnostics that don't include a code field. Some oxlint diagnostics (like parse errors) omit this field, causing code.match() to throw TypeError: Cannot read properties of undefined (reading 'match').

The bug can trigger in two code paths:

1. Line 441: When processing diagnostics with prepared source maps (e.g. Astro files)
2. Line 469: When mapping filtered diagnostics (though this is protected by isMappableOxlintDiagnostic which requires a string code)

Fix

Added a defensive null check at the start of parseRuleCode:

if (!code) return { plugin: "unknown", rule: "unknown" };

This returns a safe fallback when code is undefined, null, or empty string.

Scope

The fix is narrowly scoped to handle the missing code field gracefully without changing any other behavior. Updated the type signature from code: string to code: string | undefined to reflect reality.

Testing

• Added unit tests in packages/core/tests/oxlint-missing-code.test.ts covering all edge cases (undefined, null, empty, missing)
• Added regression test documentation in packages/react-doctor/tests/regressions/scan-resilience.test.ts
• All existing tests pass
• Parity check in progress

Closes #1635