Fix type safety and error handling in Azure Login action

[Copilot]

Problem

The codebase had unsafe error handling that could crash at runtime and weak TypeScript typing that defeated compile-time safety.

Changes

Error handling

• Add type guards before accessing error.stack and error.message properties
• Prevents runtime crashes when non-Error objects are thrown

// Before
catch (error) {
    core.setFailed(`Login failed with ${error}.`);
    core.debug(error.stack);  // Unsafe - crashes if error isn't an Error
}

// After  
catch (error) {
    const errorMessage = error instanceof Error ? error.message : String(error);
    core.setFailed(`Login failed with ${errorMessage}.`);
    if (error instanceof Error && error.stack) {
        core.debug(error.stack);
    }
}

Type safety

• Replace any types with ExecOptions interface from @actions/exec
• Affects: AzureCliLogin.executeAzCliCommand(), AzPSUtils.runPSScript()
• Enables proper IDE autocomplete and compile-time validation

Code quality

• Replace loose equality (==) with strict equality (===)
• Applies to: preCleanup environment variable check in main.ts

Original prompt

Reference: db74ff4

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[github-actions]

This PR is idle because it has been open for 14 days with no activity.