feat: add native styling support for fenced div blocks (:::classname)

[forest6511]

Summary

• Implements Issue Feature Request: Native styling support for fenced div blocks (workbook/exercise zones) #63: fenced div blocks (:::classname ... :::) can now be styled natively via YAML preset configuration, eliminating the need for post-processing scripts
• Background shading is applied to all child paragraphs, headings, lists, and table body cells
• Optional top/bottom separator borders delimit the zone boundary
• 282 → 287 tests passing (5 new unit tests)

YAML configuration example

Styles:
  FencedDivs:
    try:
      BackgroundColor: "F2F2F2"
      BorderTopColor: "AAAAAA"
      BorderTopSize: 4
      BorderBottomColor: "AAAAAA"
      BorderBottomSize: 4
    hint:
      BackgroundColor: "EBF5FB"

Markdown usage

:::try
**Exercise heading**

| Item | Fill in |
|------|---------|
| Today's event |  |

Write above, then proceed to the next step.
:::

Implementation notes

• Markdig.UseAdvancedExtensions() already includes UseCustomContainers() — no parser change needed
• TableStyle.BodyCellBackgroundColor (nullable) applies div background to table body cells
• When a div starts/ends with a table, invisible spacer paragraphs carry the separator borders
• Unrecognized class names render without visual treatment (graceful degradation)

Test plan

• AddFencedDiv_WithParagraphOnly_ShouldApplyBackgroundAndBorders
• AddFencedDiv_WithMultipleParagraphs_TopBorderOnFirstBottomBorderOnLast
• AddFencedDiv_WithNoBorderColors_ShouldApplyOnlyBackground
• AddFencedDiv_WithTableBodyCell_ShouldApplyBackgroundShading
• AddFencedDiv_WithEmptyContent_ShouldAddNoParagraphs

Closes #63

[codecov]

Codecov Report

❌ Patch coverage is 50.92025% with 80 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.06%. Comparing base (c92d362) to head (fd3c050).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...kdownToDocx.Core/OpenXml/OpenXmlDocumentBuilder.cs	58.87%	36 Missing and 8 partials ⚠️
.../MarkdownToDocx.Styling/Styling/StyleApplicator.cs	0.00%	20 Missing ⚠️
...arkdownToDocx.Styling/Models/StyleConfiguration.cs	16.66%	10 Missing ⚠️
...src/MarkdownToDocx.Core/Models/FencedDivContent.cs	37.50%	5 Missing ⚠️
...n/src/MarkdownToDocx.Core/Models/FencedDivStyle.cs	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
- Coverage   94.21%   89.06%   -5.15%     
==========================================
  Files          25       27       +2     
  Lines        1209     1372     +163     
  Branches      142      174      +32     
==========================================
+ Hits         1139     1222      +83     
- Misses         41      113      +72     
- Partials       29       37       +8     

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