fix(input): segment Thai composing text by word boundaries

[Scott-Nx]

What this fixes

When suggestions are enabled for Thai input, the entire sequence will remain one composing word because Thai does not normally use spaces between words.

This causes two visible problems:

• Suggestions are generated from the entire unsegmented sequence instead of the current Thai word.
• Pressing Space once only commits the composing text, requiring a second press to insert an actual space.

The input logic now uses locale-aware word boundaries to commit completed Thai segments while keeping only the latest segment composing.

For example, after typing ภาษาไทยดี:

• ภาษา and ไทย are committed individually.
• Only ดี remains composing.
• Suggestions are generated for the current segment instead of ภาษาไทยดี as one word.
• Pressing Space once produces exactly ภาษาไทยดี .

Completed segments are committed individually so user history and n-gram context remain correct.

LeanType's immediate Text Expander is also respected: segmentation is deferred only while the current input can still match a configured shortcut.

Reproduction

1. Select a Thai layout.
2. Enable suggestions.
3. Type multiple Thai words without spaces, such as ภาษาไทยดี.
4. Observe the suggestions and press Space once.

Before this change:

• The full sequence remain composing.
• Suggestions based on the full unsegmented sequence.
• The first Space only finished composing, requiring another press to insert a space.

After this change:

• Suggestions follow the latest Thai word segment.
• Completed segments are committed separately.
• One Space press inserts the expected space.

Testing

Tested manually with:

• Suggestions enabled
• Suggestions after single and multiple Thai word segments
• Space after segmented Thai input
• Immediate Text Expander shortcuts

Unit tests cover Thai segmentation, composing state, history and n-gram context, shortcut handling, Space insertion, and locale detection.

Source

Based on HeliBoard PR #2531, with additional changes for LeanType's Text Expander integration. The shared fixes and cleanup have since been synced back to HeliBoard.