Refactor StatusEditor text handling into TextState/TextService and add tests

[Dimillian]

Motivation

• Reduce the monolithic ViewModel responsibilities by extracting text setup and processing into dedicated helpers.
• Improve readability and testability of text-related logic by breaking large functions into smaller, focused pieces.
• Provide a stable API for the editor view to bind to text state via a Binding instead of direct NSMutableAttributedString mutation.
• Add unit tests to validate core text processing behavior in isolation.

Description

• Extracted text state into StatusEditor.TextState (Editor/TextState.swift) holding statusText, mentionString, urlLengthAdjustments, suggestion range and backup text.
• Introduced StatusEditor.TextService (Editor/TextService.swift) that encapsulates initial text setup, insertion/replacement helpers, and processText logic (highlights, URL handling, suggestions).
• Refactored StatusEditor.ViewModel to use textState + textService, added statusTextBinding for the editor view, and replaced large text manipulation functions with small delegating methods like updateStatusText and applyTextChanges.
• Updated EditorView to use the new statusTextBinding and added a StatusKit test target with TextServiceTests that exercise hashtag suggestion, suggestion reset, URL length adjustment, and initial reply mention generation.

Testing

• Added StatusKitTests/TextServiceTests.swift with unit tests for TextService behaviors including suggestion detection and URL length adjustments.
• Added a StatusKit test target in Packages/StatusKit/Package.swift so tests are discoverable by the package test runner.
• No automated tests were executed in this environment, so no pass/fail results are available here.
• Tests are written to be run via the normal Xcode / Swift Package Manager test workflows (e.g. xcodebuild or swift test).

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Codex Task