Fix: Replace missing indent icon names in RichText control

[Copilot]

Description

Fixes the issue where the increase/decrease indent buttons in the RichText control's style pane were displaying as missing icon boxes instead of proper icons.

Problem

When users opened the RichText control, authored text, and clicked the ellipsis in the style ribbon to open the style pane, the increase and decrease indent controls showed empty boxes (□) instead of the expected icons.

Root Cause

The component was using icon names IncreaseIndentLegacy and DecreaseIndentLegacy which do not exist in the Fluent UI MDL2 icon font set that ships with @fluentui/react version 8.106.4.

Solution

Updated the icon names to use the correct Fluent UI MDL2 icons:

• IncreaseIndentLegacy → IncreaseIndentText
• DecreaseIndentLegacy → DecreaseIndentText

These icons are part of the official Fluent UI MDL2 icon set and display horizontal lines (representing text) with directional arrows to indicate indent direction.

Changes

src/controls/richText/RichTextPropertyPane.tsx

• Updated increase indent button to use IncreaseIndentText icon
• Updated decrease indent button to use DecreaseIndentText icon

src/controls/iconPicker/IconNames.ts

• Added IncreaseIndentText to the available icons list
• Added DecreaseIndentText to the available icons list
• Retained legacy icon names for backward compatibility

Testing

• ✅ Build completed successfully with no errors or warnings
• ✅ Verified icon names exist in Fluent UI MDL2 icon set
• ✅ Confirmed no other components use the legacy icon names

Impact

Users will now see proper indent control icons in the RichText style pane instead of missing icon boxes.

References

• Fluent UI MDL2 Icons Documentation
• IncreaseIndentText Icon Preview
• DecreaseIndentText Icon Preview

Fixes #ISSUE_NUMBER

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Richtext indent controls missing icons</issue_title>
<issue_description>#### Category
[ ] Enhancement

[X ] Bug

[ ] Question

Version

Please specify what version of the library you are using: [ 3.22.0 ]

Expected / Desired Behavior / Question

The icons for increase\decrease indent should load

Observed Behavior

Both these icons (in the style pane) are showing as a box (missing)
IncreaseIndentLegacy
DecreaseIndentLegacy

Steps to Reproduce

Author some text in the webpart and highlight it. Then click the ellipsis in the style ribbon to open the style pane.
</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #2045

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