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

[schani]

The bug

When --src pointed at a directory, --src-lang schema was silently
ignored for .json files in that directory. samplesFromDirectory() (used
only for directory sources) classified every file purely by its extension —
.json/.url were always parsed as plain JSON samples, .schema was
always parsed as JSON Schema — with no way to override this via
--src-lang. This meant a directory of *.schema.json files passed with
--src-lang schema generated interfaces from the raw JSON structure of the
schema documents ($schema, $id, title, properties, an Age helper
type, etc.) instead of from the schema they describe. Passing the exact same
file directly (not through a directory) worked correctly, because the
non-directory code path (typeSourcesForURIs) does switch on
options.srcLang.

Root cause

getSources() in src/index.ts calls samplesFromDirectory(dataDir, options.httpHeader) without ever passing options.srcLang through, and
samplesFromDirectory/its inner readFilesOrURLsInDirectory had no
parameter for it — extension-based detection was the only path.

The fix

• samplesFromDirectory now accepts an optional sourceLanguage parameter.
• getSources passes options.srcLang into it.
• When sourceLanguage === "schema", .json/.url files inside a
  directory are treated as JSON Schema sources (matching what happens when
  the same file is passed directly, without going through a directory).
• When --src-lang is not explicitly "schema", the previous
  extension-based auto-detection (.json/.url → JSON, .schema → schema,
  .gqlschema/.graphql → GraphQL) is unchanged, so existing directory
  fixtures relying on that default behavior are unaffected.

Test coverage

Added test/unit/directory-source-language.test.ts, which:

1. Reproduces the bug end-to-end via the CLI's main() entry point: runs
   --src-lang schema once on a single schema file and once on a directory
   containing only that file, and asserts the generated TypeScript is
   byte-identical between the two invocations (this fails on unfixed code,
   since the directory case previously produced JSON-derived output instead
   of schema-derived output).
2. Confirms the default (no --src-lang) directory-scanning behavior still
   auto-detects .schema files as JSON Schema by extension, guarding
   against a regression of the pre-existing behavior.

This was placed in test/unit/ rather than as an end-to-end JSON/JSON
Schema fixture because the bug is specifically in the CLI's
directory-scanning/dispatch logic in src/index.ts (which extension maps to
which source kind, given --src-lang), a layer the fixture harness (which
drives the library API directly rather than the CLI's argument/directory
handling) does not exercise.

Verification

• npm run build — passes.
• npm run test:unit — 172/172 tests pass (including the two new tests).
• QUICKTEST=true FIXTURE=schema-typescript script/test — 70/70 tests pass,
  confirming the fix doesn't affect ordinary (non-directory,
  non---src-lang-schema-on-directory) schema handling.
• Manually re-ran the original repro:
  node dist/index.js --src-lang schema --lang ts --src <dir-with-person.schema.json>
  now produces the correct schema-derived interface
  (age?: number; name: string; [property: string]: unknown;) instead of
  the previous JSON-derived interface with bogus $schema/$id/title/
  properties/Age fields.
• CI will additionally validate the broader fixture matrix across all
  languages.

Fixes #1278

🤖 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.