Fix SSOv2 HttpClient null bug, remove pointless try-catches, minor code quality

[Copilot]

Several issues identified during codebase review — ranging from a silent null-assignment bug to noise code patterns.

Bug: SSOv2._httpClient never assigned when caller provides a client

// Before: custom httpClient silently discarded; _httpClient stays null on first use → NullReferenceException
if (httpClient == null)
{
    _httpClient = new HttpClient();
}

// After
_httpClient = httpClient ?? new HttpClient();

Additionally, _httpClient was static, meaning all SSOv2 instances shared one client — a second instantiation would silently overwrite it. Changed to an instance field.

Code quality

• Removed two try { ... } catch (Exception) { throw; } blocks in APIBase.RequestAsync and APIBase.ProcessSuccess — they re-throw unchanged with no side effects.
• TRANQUILITY_ESI_BASE / SERENITY_ESI_BASE promoted from mutable static fields to const.
• CheckAuth was throwing new ArgumentNullException() with no message or parameter name; replaced with ArgumentException carrying a descriptive message covering all validated inputs.
• <PackageLicenseUrl></PackageLicenseUrl> removed from .csproj — deprecated and already superseded by the existing <PackageLicenseExpression>.

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.