Forward initialDirectory and dialogTitle to native on Darwin

[vicajilau]

Summary

• pickFile(), pickFiles(), and getDirectoryPath() in file_picker_darwin accepted initialDirectory (and dialogTitle for getDirectoryPath()) but never included them in the method channel arguments map, so they never reached native code.
• The native macOS implementation already read args["initialDirectory"]/args["dialogTitle"] correctly in all four of its handlers, it was simply never receiving them.
• Added Dart unit tests asserting both arguments are forwarded for pickFile(), pickFiles(), and getDirectoryPath().
• Bumped file_picker_darwin to 2.1.2 (patch, bug fix) with a changelog entry.

Fixes #2216

Test plan

• flutter test test/file_picker_darwin_test.dart (12/12 passing, including the 2 new tests)
• dart analyze lib test (no issues)
• dart format applied

[navaronbracke]

LGTM, although I would also cross check Windows / Linux / Android

[vicajilau]

Good call @navaronbracke!! I checked all three.

Windows and Linux are fine end to end. initialDirectory reaches the actual native call in both (arena.pcwstr(initialDirectory) on Windows, the D-Bus request encoding on Linux), and Windows was already confirmed working on a real machine earlier in #2216.

Android has the same Dart side gap this PR fixes for Darwin (initialDirectory/dialogTitle aren't added to the method channel arguments for pick/getDirectoryPath), but it goes further there. The native Kotlin side never reads initialDirectory for ACTION_OPEN_DOCUMENT, ACTION_OPEN_DOCUMENT_TREE, or the ACTION_GET_CONTENT variants either, only saveFile supports it today. Making it work on Android means adding native support for turning a plain path into a SAF document tree URI, not just forwarding an argument, so it's a bigger, separate piece of work rather than something this PR should take on. I'll file a follow up issue for that instead of scope creeping this one.