Fix TextBox UI Automation fragment root

[KSaiSivani]

Fixes #15043

Proposed changes

• Return the TextBoxBaseAccessibleObject as its own UI Automation fragment root.
• Prevent IRawElementProviderFragment.get_FragmentRoot from returning S_OK with a null provider for text controls.
• Add COM-level regression coverage before and after control handle creation.

Customer Impact

• Prevents UI Automation clients from receiving an invalid null fragment root from TextBox, MaskedTextBox, and UpDown edit controls.
• Prevents the application hangs reported when JAWS invokes TextPattern.RangeFromPoint.

Regression?

• No

Risk

• Low

Test methodology

• Added a COM-level test verifying that get_FragmentRoot returns S_OK with a valid provider.
• Verified the fragment root before and after control handle creation.
• Ran all TextBoxBaseAccessibleObjectTests: 83 passed.
• Ran the related ToolStrip fragment-root tests: 53 passed.

Test environment(s)

• .NET SDK 11.0.100-preview.6.26359.118

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.24166%. Comparing base (b0a7c95) to head (5a9c64c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##                main      #15058   +/-   ##
=============================================
  Coverage   37.24166%   37.24166%           
=============================================
  Files            246         246           
  Lines           9774        9774           
  Branches        1029        1029           
=============================================
  Hits            3640        3640           
  Misses          5970        5970           
  Partials         164         164           

Flag	Coverage Δ
Debug	37.24166% <ø> (ø)
production	39.36526% <ø> (ø)
test	20.64923% <ø> (ø)
unit	39.36526% <ø> (ø)

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