Fix nil pointer dereference in explainRedirectAndImpliedFormat

[Copilot]

explainRedirectAndImpliedFormat panics when GetSourceFileByPath returns nil for a given path. The nil *ast.SourceFile is assigned to the ast.HasFileName interface, then .Path() is called on it.

• Add nil guard after GetSourceFileByPath, returning nil (empty diagnostics) early when the source file isn't found
• Add test case that reproduces the crash: on a case-sensitive filesystem, two root files with different casing (/src/MyFile.ts and /src/myFile.ts) where only one exists, triggering a file casing diagnostic whose file path references the missing file

[DanielRosenwasser]

@copilot try to figure out a test case for this.

[Copilot]

@copilot try to figure out a test case for this.

Added a test in 4e3a3adf. The scenario: on a case-sensitive filesystem, two root files with different casing (/src/MyFile.ts and /src/myFile.ts) where only the lowercase version exists. This triggers a file casing diagnostic in the include processor whose file path points to the missing /src/MyFile.ts. When explainRedirectAndImpliedFormat is called during diagnostics computation, GetSourceFileByPath returns nil for that path, causing the nil pointer dereference. Verified the test fails without the fix and passes with it.