Support autocomplete when the caret is in the middle of text

[FuJacob]

Summary

Cotabby should support inline autocomplete when the caret is inside existing text, not only when typing at the end of a field.

Problem

Today the suggestion flow is optimized for appending text after the current caret. When the user places the caret in the middle of a paragraph, email, message, or document, suggestions need to respect both the text before the caret and the text after it. Without explicit middle-of-text support, completions can feel unsafe or get suppressed because accepting a suggestion may conflict with the existing trailing text.

Desired behavior

• If the caret is in the middle of editable text, Cotabby can still generate a useful continuation at the caret.
• Ghost text renders next to the caret without visually colliding with the existing trailing text.
• Accepting with Tab inserts only the suggested text at the caret and preserves all existing text after the caret.
• Suggestions should avoid duplicating or contradicting the trailing text already present after the caret.

Acceptance criteria

• Detect and allow eligible focus snapshots where trailingText is non-empty.
• Include bounded trailing context in the request/prompt so the engine knows what follows the caret.
• Normalize or reject suggestions that simply repeat the existing trailing text.
• Ensure acceptance/reconciliation inserts at the caret without deleting or moving the trailing text.
• Add focused tests around request construction, normalization/reconciliation, and acceptance behavior for middle-of-text cases.

Implementation notes

This likely touches SuggestionAvailabilityEvaluator, SuggestionRequestFactory, prompt rendering, SuggestionSessionReconciler, and acceptance/insertion handling. The main tradeoff is safety versus usefulness: we should start conservative, especially in hosts with flaky AX selection/trailing-text reporting, and expand once insertion behavior is proven stable across native AppKit, Chromium, and Electron fields.