fix(schema-input): report clean fetch error for missing schema paths

[schani]

Bug

--src-lang schema crashed with an unhelpful internal-error panic when given
a source path that doesn't exist:

$ node dist/index.js --src-lang schema --lang typescript --just-types --src '*.json'
Error: Internal error: Defined value expected, but got undefined.

This typically bites Windows/PowerShell users, whose shell doesn't expand
--src *.json the way POSIX shells do, so the literal, non-existent string
*.json is passed through to quicktype. For comparison, --src-lang json
with the same missing-path input gives a clean ENOENT error instead of a
panic.

Root cause

JSONSchemaStore.get (packages/quicktype-core/src/input/JSONSchemaStore.ts)
swallows the fetch error for a missing file and returns undefined.
Resolver.resolveVirtualRef (packages/quicktype-core/src/input/JSONSchemaInput.ts)
then calls schemaFetchError(base, virtualRef.address) to report the
failure — but virtualRef has no address in this case, and Ref.address
panics via defined() when addressURI is undefined. That panic is what
surfaces as "Internal error: Defined value expected, but got undefined."

Fix

Use virtualRef.toString() instead of virtualRef.address when building the
fetch-error message. toString() doesn't require an address (it degrades to
an empty string for the address portion), so building the error message no
longer panics. The CLI now reports a normal diagnostic:

$ node dist/index.js --src-lang schema --lang typescript --just-types --src '*.json'
Error: Could not fetch schema #, referred to from *.json#.

This is a minimal, one-line change scoped to the crash itself. CLI-side glob
expansion (the other defect discussed on the issue) is out of scope for this
PR.

Test coverage

Added test/unit/missing-schema-path.test.ts, a regression unit test that
drives JSONSchemaInput/FetchingJSONSchemaStore/quicktype() directly
(the same pattern used by test/unit/windows-schema-paths.test.ts for issue
#2869) to assert that a schema source pointing at a nonexistent path (a
literal wildcard, simulating what PowerShell passes through) rejects with a
normal "Could not fetch schema ..." error rather than throwing/panicking.
This is a unit test rather than a fixture test because the bug is about
error-handling/messaging for a missing input file, not about generated
output for valid input — exactly the kind of thing fixtures can't express.

Confirmed the test fails against the unfixed code (the panic message) and
passes after the fix.

Verification

• npm run build passes.
• npm run test:unit — all 164 unit tests pass, including the new
  regression test.
• Manually re-ran the exact repro from the issue/triage comment and confirmed
  the internal-error panic no longer occurs; a clean fetch-error message is
  reported instead.
• No fixture test was added since this change does not affect generated
  output for any valid input — CI will additionally validate the full
  fixture suite.

Fixes #2812

🤖 Generated with Claude Code