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

[schani]

Bug

quicktype --src-lang schema <directory>/ -o Models.ts ignored --src-lang
entirely: every .json file inside the directory was parsed as a JSON
data sample (interfaces mirroring the schema document's own keys like
type/required/properties) instead of as a JSON Schema document.
Passing the exact same file directly (--src-lang schema schemas/alert_schema.json) worked correctly, so the bug was specific to the
directory-scanning code path.

Root cause

In src/index.ts, single-file/URL inputs go through typeSourcesForURIs(),
which correctly switches on options.srcLang to decide whether a source is
JSON data or a JSON Schema document. Directory inputs, however, go through
getSources() -> samplesFromDirectory(), which classified every file
purely by its extension (.json/.url -> JSON data, .schema -> JSON
Schema, .gqlschema/.graphql -> GraphQL) without ever consulting
options.srcLang. As a result --src-lang schema had no effect on .json
files found inside a directory.

Fix

samplesFromDirectory() now takes the source language as a parameter. When
it is "schema", .json/.url files inside the directory are constructed
as kind: "schema" sources (matching what typeSourcesForURIs() already
does for explicit file arguments) instead of kind: "json" data samples.
Behavior for .schema, .gqlschema, .graphql files, and the default
(json) source language is unchanged.

Test coverage

Added test/unit/directory-schema-input.test.ts, which drives the actual
CLI entry point (main() in src/index.ts) against a temp directory
containing two .json JSON Schema files with --src-lang schema, and
asserts the generated TypeScript interfaces reflect the schemas' declared
properties rather than the schema documents' own top-level keys. This test
fails on unfixed code and passes after the fix. A CLI-level unit test was
used (rather than the test/fixtures.ts language-fixture harness) because
this bug lives specifically in CLI argument/directory-scanning logic
(getSources/samplesFromDirectory), a code path the fixture harness does
not exercise since it drives the Input/InputData API directly with a
single source.

Verification

• npm run build passes.
• npm run test:unit: all 164 unit tests pass, including the new regression
  test.
• Manually re-ran the issue's exact repro (--src-lang schema schemas/ -o Models.ts against a directory containing the issue's alert_schema.json)
  and confirmed the output now correctly reflects the schema's properties
  (alert_type_id, issue_id, options) instead of the schema document's
  own keys.
• Ran QUICKTEST=true FIXTURE=schema-typescript script/test (67 tests) as a
  sanity check for schema-handling regressions elsewhere; all passed.
• CI will run the full fixture matrix across all target languages.

Fixes #1910

🤖 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.