Use theme tabRow color as acrylic tint (#19604)

[EclipseAditya]

Summary of the Pull Request

When useAcrylicInTabRow is enabled, the theme's tabRow.background color is now used as the acrylic tint instead of being ignored.

References and Relevant Issues

• Fixes #19604

Detailed Description of the Pull Request / Additional comments

Previously, the code had mutually exclusive branches for handling useAcrylicInTabRow and theme tabRow.background. When acrylic was enabled, the theme color was never evaluated.

This PR extracts the theme's tabRow color first, then uses it as the acrylic tint if set, falling back to the default if no theme color is specified.

Validation Steps Performed

• Tested with useAcrylicInTabRow: true and custom tabRow.background - acrylic now shows the theme color as tint

settings.json

{
    "$help": "https://aka.ms/terminal-documentation",
    "$schema": "https://aka.ms/terminal-profiles-schema",
    
    "theme": "BugTestTheme",
    "useAcrylicInTabRow": false,
    
    "themes": [
        {
            "name": "BugTestTheme",
            "tabRow": {
                "background": "#00FF00CC",
                "unfocusedBackground": "#FF0000CC"
            },
            "window": {
                "applicationTheme": "dark"
            }
        }
    ],
    
    "profiles": {
        "defaults": {
            "font": {
                "face": "Cascadia Mono"
            }
        },
        "list": [
            {
                "commandline": "%SystemRoot%\\System32\\WindowsPowerShell\\v1.0\\powershell.exe",
                "guid": "{61c54bbd-c2c6-5271-96e7-009a87ff44bf}",
                "name": "Windows PowerShell"
            }
        ]
    },
    
    "actions": []
}

with "useAcrylicInTabRow": true

with "useAcrylicInTabRow": false

PR Checklist

• [x] Closes #19604

[EclipseAditya]

Hi @DHowett, Can you please review this PR :).