fix: Windows terminal fallback defaults to PowerShell 7 instead of legacy PS 5.1

Fixes two bugs causing Windows users with PS7 configured to get legacy
PowerShell 5.1, resulting in UTF-8 encoding issues (mojibake):

1. TerminalProcess.ts: Fix incorrect VS Code config API call that always
   returned null. Changed from getConfiguration("terminal.integrated.defaultProfile")
   .get("windows") to getConfiguration("terminal.integrated").get("defaultProfile.windows").

2. shell.ts: Change PowerShell fallback from legacy PS 5.1 to PS 7 when
   profile name includes "powershell" but has no explicit path or source,
   since VS Code auto-discovers PS7 profiles without those properties.

Closes #11958

src/integrations/terminal/TerminalProcess.ts

@@ -97,8 +97,8 @@ export class TerminalProcess extends BaseTerminalProcess {
 
 		// Execute command
 		const defaultWindowsShellProfile = vscode.workspace
-			.getConfiguration("terminal.integrated.defaultProfile")
-			.get("windows")
+			.getConfiguration("terminal.integrated")
+			.get<string>("defaultProfile.windows")
 
 		const isPowerShell =
 			process.platform === "win32" &&

src/utils/__tests__/shell.spec.ts

@@ -139,11 +139,11 @@ describe("Shell Detection Tests", () => {
 			expect(getShell()).toBe("C:\\Program Files\\PowerShell\\7\\pwsh.exe")
 		})
 
-		it("falls back to legacy PowerShell if profile includes 'powershell' but no path/source", () => {
+		it("falls back to PowerShell 7 if profile includes 'powershell' but no path/source", () => {
 			mockVsCodeConfig("windows", "PowerShell", {
 				PowerShell: {},
 			})
-			expect(getShell()).toBe("C:\\Windows\\System32\\WindowsPowerShell\\v1.0\\powershell.exe")
+			expect(getShell()).toBe("C:\\Program Files\\PowerShell\\7\\pwsh.exe")
 		})
 
 		it("uses WSL bash when profile indicates WSL source", () => {

src/utils/shell.ts

@@ -205,8 +205,9 @@ function getWindowsShellFromVSCode(): string | null {
 			// If the profile is sourced from PowerShell, assume the newest
 			return SHELL_PATHS.POWERSHELL_7
 		}
-		// Otherwise, assume legacy Windows PowerShell
-		return SHELL_PATHS.POWERSHELL_LEGACY
+		// Otherwise, default to PowerShell 7 (modern) since VS Code auto-discovers
+		// PS7 profiles without setting an explicit path or source property
+		return SHELL_PATHS.POWERSHELL_7
 	}
 
 	// If there's a specific path, return that immediately