[api] Report project reference diagnostics on programs without a config file

[Christopher Plieger (cplieger)]

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Fixes #64629

A program created through createPrograms has no configuration file, so its ParsedCommandLine.ConfigFile is nil. Any project reference diagnostic on such a program crashes the API server with a nil pointer dereference in CreateDiagnosticAtReferenceSyntax.

Impact: a tool that builds its own program for a repository that references a non-composite project kills the whole API server process, losing every open snapshot, and the tool sees only an EOF. That is an ordinary monorepo setup, where tsc -p reports TS6306 and keeps going, so an analyzer on the API stops on that repository instead of analyzing it.

CreateDiagnosticAtReferenceSyntax now returns nil when ConfigFile is nil, and its caller in verifyProjectReferences already falls back to a compiler diagnostic in that case.

TestCreateProgramReportsNonCompositeProjectReference creates such a program with a reference to a non-composite project and expects TS6306 as its only program diagnostic, and it panics without the fix.

I met this while building deadset-ts, the TypeScript analyzer of deadset, on the TypeScript 7 API. That work hit a small set of related defects, which is why a few reports come from me.

An AI coding agent wrote this patch. I have read, built and tested it and will handle the review.

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused fix correctly uses the existing compiler-diagnostic fallback and includes regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Prevents API crashes when synthetic programs without config files report project-reference diagnostics.

Changes:

• Adds a nil guard before accessing ConfigFile.
• Adds regression coverage for TS6306 reporting.

File	Description
tsc/​internal/​tsoptions/​tsconfigparsing.go	Falls back when reference syntax is unavailable.
tsc/​internal/​api/​session_createprogram_test.go	Verifies non-composite reference diagnostics.

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