Add unit tests for mushaf layout JSON export (follow-up to #620)

[hammadxcm]

Description

Adds unit tests for Exporter::ExportMushafLayout#export_json (the JSON layout export shipped in #620, closing #257). When that PR was opened the codebase had no exporter tests and I couldn't run anything end-to-end, so the original implementation went in with only static checks. This locks the JSON shape down with a real Minitest spec.

Coverage (9 cases / 31 assertions):

• export_json returns the JSON directory path it wrote to.
• info.json carries the mushaf metadata (name, number_of_pages, lines_per_page, font_name).
• One <page_number>.json file is written per page.
• surah_name line carries surah_number, type, and centered alignment (and omits ayah-only keys).
• basmallah line is centered with no word data.
• ayah line carries data (word text), first_word_id / last_word_id, and justified alignment.
• ayah falls back to justified when no MushafLineAlignment row exists for the line.
• Word text honors Mushaf#text_type_method — switching the method to code_v1 swaps the exported text to the glyph codes.
• Words on a line are sorted by word_index even when the underlying records are out of order.

Test style

Follows the convention already established by test/services/resources/ — standalone Minitest, PORO/Struct stubs for the ActiveRecord models (Mushaf, MushafPage, MushafLineAlignment, plus fake page/word/alignment records), no Rails boot, no database. Stubs are guarded with unless Object.const_defined?(...) so they don't clobber real constants if Rails were loaded.

Related Issue

Follow-up to #257 (closed by #620, which shipped the feature without tests).

Motivation and Context

The JSON export is now consumed by downstream apps — pinning its shape with tests prevents accidental regressions (e.g., a stray rename of a key or a change to the line-type vocabulary). The same fake-AR pattern can be reused for the SQLite export and other exporter tests in the future.

How Has This Been Tested?

$ ruby -Itest test/lib/exporter/export_mushaf_layout_test.rb
9 runs, 31 assertions, 0 failures, 0 errors, 0 skips

Also ruby -c clean and RuboCop (Layout/Lint) clean.

Screenshots (if appropriate):

N/A — test-only change.

[hammadxcm]

@naveed-ahmad Follow Up PR on unit tests.