Issue 2715

[yashas-hm]

Description

Adds support for custom inline embeds (mentions, hashtags, etc.) to declare their character length for the platform keyboard, fixing TextCapitalization.sentences and word/sentence boundary detection
around embeds.

Problem: All embeds are sent to the platform as a single \uFFFC character. The OS keyboard cannot detect sentence endings (e.g. @JohnDoe. Next sentence) because it only sees \uFFFC. --> not
enough context for capitalization or word boundaries.

Solution:

• Added int get length property to EmbedBuilder (defaults to 1, fully backward compatible)
• Added internal OffsetMapping layer at the platform text-input boundary that translates between document offsets (embeds = length 1) and expanded-text offsets (embeds = toPlainText().length)
• The Delta/OT layer is unchanged --> embeds remain length 1 internally

Usage: Override toPlainText() and length on your EmbedBuilder:

class MentionBuilder extends EmbedBuilder {
  @override
  String get key => 'mention';

  @override
  bool get expanded => false;

  @override
  String toPlainText(Embed node) => '@${node.value.data}';

  @override
  int get length => toPlainText(...).length;
}

Files changed:
- lib/src/editor/embed/embed_editor_builder.dart --> added int get length => 1;
- lib/src/editor/raw_editor/offset_mapping.dart --> new internal OffsetMapping utility
- lib/src/controller/quill_controller.dart --> cached offsetMapping + expandedTextEditingValue
- lib/src/editor/raw_editor/raw_editor_state_text_input_client_mixin.dart --> use expanded text for platform I/O
- test/editor/offset_mapping_test.dart --> 17 unit tests
- CHANGELOG.md --> added entry under [Unreleased]


## Related Issues
Fixes #2715 
Closes #2715 

## Type of Change

<!---
Check the boxes that apply with x and leave the others empty. For example:
- [x] ✨ **New feature:** Adds new functionality without breaking existing features.
- [ ] 🛠️ **Bug fix:** Resolves an issue without changing current behavior.
-->

- [x] ✨ **Feature:** New functionality without breaking existing features.
- [ ] 🛠️ **Bug fix:** Resolves an issue without altering current behavior.
- [ ] 🧹 **Refactor:** Code reorganization, no behavior change.
- [ ] ❌ **Breaking:** Alters existing functionality and requires updates.
- [x] 🧪 **Tests:** New or modified tests
- [ ] 📝 **Documentation:** Updates or additions to documentation.
- [ ] 🗑️ **Chore:** Routine tasks, or maintenance.
- [ ] ✅ **Build configuration change:** Build/configuration changes.