fix(cli): honor --src-lang schema for directory sources

[schani]

Bug

quicktype --lang <lang> --src-lang schema --src <directory> produced invalid
output when the directory contained schema files named with a .json
extension: the generated types were full of schema-document metadata
($schema, $id, title, properties, required) instead of the type the
schema actually describes. Passing the exact same files individually with
--src-lang schema --src file1.json --src file2.json ... worked correctly.

Root cause

samplesFromDirectory in src/index.ts classifies each file in a --src
directory purely by file extension (.json -> JSON sample, .schema -> JSON
Schema), and never consults options.srcLang. The non-directory code path
(typeSourcesForURIs) already branches correctly on options.srcLang
("json" vs "schema") regardless of extension, but that branching was never
threaded through to the directory-scanning path, so a directory of
--src-lang schema files named *.json was always parsed as JSON samples
rather than JSON Schema documents.

Fix

Thread srcLang from options into samplesFromDirectory. When
srcLang === "schema", .json files found in the directory are now treated
as schema sources (kind: "schema") instead of JSON sample sources, matching
the behavior of the per-file invocation path. .schema, .url, and GraphQL
handling, plus the default --src-lang json directory behavior, are
unchanged.

Test coverage

Added test/unit/schema-directory-src-lang.test.ts: writes three
cross-referencing ($ref) JSON Schema documents (location.json,
person.json, product.json) to a temp directory, generates Rust from the
directory with --src-lang schema, generates Rust from the same three files
passed individually with --src-lang schema, and asserts the two outputs are
identical. This test fails on unfixed src/index.ts (directory output
contains schema-metadata structs) and passes with the fix.

This is CLI-level directory-scanning logic in src/index.ts, not
per-language generated-output shape, so it isn't expressible through the
test/inputs/{json,schema}/ fixture harness (which drives quicktype with
individual schema/JSON files, not a --src <directory> invocation with a
chosen --src-lang); a unit test is the appropriate mechanism here, matching
the existing precedent of test/unit/windows-schema-paths.test.ts for a
similar directory-handling issue.

Verification

• npm run build passes.
• npm run test:unit — all 164 unit tests pass, including the new one.
• Manually re-ran the original repro (`--lang rust --src-lang schema --src ` vs. `--src-lang schema --src file1 --src file2 --src file3` individually) — outputs are now identical and correctly reflect the schemas' modeled types (`Location { latitude, longitude }`, etc.) with no schema-metadata fields.
• Ran the full rust fixture suite locally (QUICKTEST=true FIXTURE=rust script/test, 57/57 tests) to confirm no regressions in Rust codegen;
  remaining language fixtures are left to CI.

Fixes #2634

🤖 Generated with Claude Code

[github-actions]

No generated-output differences

✅ Generated outputs are unchanged between the PR base and head revisions.

[github-actions]

No generated-output differences

✅ Generated outputs are unchanged between the PR base and head revisions.