Fix: Add accessible label to ability test payload textarea in Abiliti…

[Trushiv04]

What?

Closes #648

Adds an accessible label to the JSON payload <textarea> in the Abilities Explorer test screen.

Why?

The test input <textarea id="ability-test-payload"> had no associated label, aria-label, or aria-labelledby. Screen readers announced only "text area" with no indication of the field's purpose, which is a WCAG 2.1 AA violation (SC 1.3.1 and SC 4.1.2).

How?

Added a visually hidden <label class="screen-reader-text"> before the textarea, linked via the for attribute matching the existing id. This uses WordPress's standard screen-reader-text pattern, so there is no visual change the label is only exposed to assistive technology.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude
Used for: Identifying the issue location, and verifying coding standards. Final implementation and testing were done by me.

Testing Instructions

1. Navigate to Tools → Abilities Explorer and open any ability's test view
2. Focus the JSON payload textarea with a screen reader
3. Confirm it announces "Ability test input (JSON)" instead of just "text area"
4. Confirm there is no visual change to the textarea

Screenshots or screencast

Before

(screen reader announces "You are currently on a text area")

After

(screen reader announces "Ability test input (JSON), edit text")

Changelog Entry

Fixed - Added an accessible label to the ability test payload textarea in the Abilities Explorer.

[github-actions]

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Co-authored-by: Trushiv04 <trushiv@git.wordpress.org>

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.57%. Comparing base (35be266) to head (666d797).

Files with missing lines	Patch %	Lines
...udes/Experiments/Abilities_Explorer/Admin_Page.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #649      +/-   ##
=============================================
- Coverage      74.58%   74.57%   -0.01%     
  Complexity      1754     1754              
=============================================
  Files             85       85              
  Lines           7547     7548       +1     
=============================================
  Hits            5629     5629              
- Misses          1918     1919       +1     

Flag	Coverage Δ
unit	74.57% <0.00%> (-0.01%)	⬇️

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