fix(rust): don't panic when there are no top-level types

[schani]

What was broken

Generating Rust from a TypeScript source that has no #TopLevel-marked
interface (the common case — #TopLevel is an internal quicktype
convention, not something users write) crashed with:

Error: Internal error: Defined value expected, but got undefined.

Repro (from the issue, using VS Code's debugProtocol.d.ts):

node dist/index.js --lang rs --derive-debug --density dense --visibility private \
  --out debugProtocolTypes.rs --src-lang typescript --src debugProtocol.d.ts

C# generation from the same input succeeds, confirming this was a
Rust-renderer-specific bug rather than a TypeScript-input or CLI issue (some
issue commenters chased an unrelated -- argument-parsing red herring).

Root cause

When the TypeScript-to-JSON-Schema conversion finds no #TopLevel marker, it
falls back to uris: ["#/definitions/"], which does not name any specific
top-level type — the renderer's this.topLevels map ends up legitimately
empty even though the type graph itself is non-empty. RustRenderer's
emitLeadingComments() unconditionally did:

const topLevelName = defined(mapFirst(this.topLevels)).getCombinedName();

defined() panics whenever its argument is undefined, so an empty
topLevels map crashed the whole run. Other renderers (e.g. C#'s
emitDefaultLeadingComments) don't assume a top level exists for their
default leading comment, so only Rust was affected.

Fix

In packages/quicktype-core/src/language/Rust/RustRenderer.ts,
emitLeadingComments() now checks whether a top level exists and simply
skips the top-level-specific example comment when it doesn't, instead of
panicking:

const topLevel = mapFirst(this.topLevels);
if (topLevel === undefined) return;

const topLevelName = topLevel.getCombinedName();

This is a narrowly scoped change — no other renderer or shared code paths
were touched.

Test coverage

This condition (empty topLevels with a non-empty type graph) isn't
reachable through the JSON/JSON-Schema fixture harness, since fixture
schemas always assign a concrete top-level name. Per repo convention this
makes a test/unit/ test the right coverage, following the same pattern as
test/unit/typescript-input.test.ts and
test/unit/kotlin-klaxon-empty-object-map.test.ts.

Added test/unit/rust-empty-top-levels.test.ts, which drives quicktype()
directly with a JSONSchemaInput built the same way
quicktype-typescript-input builds one when no #TopLevel marker is
present (uris: ["#/definitions/"]), asserting:

• Rust renders without throwing when the schema's definitions produce no
  top-level type at all.
• Rust still renders named types correctly when a top level is present
  (regression guard).

Verification

• npm run build passes.
• npx vitest run test/unit/rust-empty-top-levels.test.ts test/unit/typescript-input.test.ts — all pass.
• Re-ran the exact repro from the issue against the built dist/index.js
  with the real VS Code debugProtocol.d.ts: previously crashed with the
  "Defined value expected" panic, now exits 0 and writes valid Rust source.
• QUICKTEST=true FIXTURE=rust script/test (full Rust fixture suite, using
  the local rustc/cargo toolchain) — run locally to confirm no
  regressions; CI will also validate this on the PR.

Fixes #1629

🤖 Generated with Claude Code