Resolve the AI tool authorization caller through IUserAccessor

[fsfh60]

What changed

AI tool authorization resolved the current caller from IHttpContextAccessor. HttpContext is documented as unreliable inside SignalR hub invocations — long-lived transports such as WebSockets, backplane-delivered invocations, and hosted SignalR services all run hub methods outside the request that opened the connection, so the accessor is frequently null there.

Because FunctionInvocationAICompletionServiceHandler passed that null principal straight to IAIToolAccessEvaluator, hosts that enforce permissions denied every tool for hub-based chat. The user simply got an answer with no tools and no explanation.

The fix

New IAIUserAccessor abstraction, backed by an AsyncLocal implementation that mirrors the HttpContextAccessor pattern:

1. If a principal was pushed onto the current asynchronous flow, it wins.
2. Otherwise fall back to HttpContext.User for ordinary HTTP requests.

AIChatHubCore and ChatInteractionHubBase now publish Context.User for the duration of every invocation via a private RunInScopeAsync wrapper. ExecuteInScopeAsync stays protected virtual, so existing host overrides keep working untouched.

Null policy

This is the one behavior change worth reviewing:

• null means there is no caller at all — a background task, a workflow, a recipe. Authorization is skipped and every tool stays available, because trusted server-side code is not a security boundary.
• An unauthenticated caller is not null. Hubs and HTTP requests always provide a ClaimsPrincipal with an unauthenticated identity, so the evaluator still runs and hosts can grant or deny tools for anonymous users as they see fit.

This makes the framework agree with the Orchard Core integration, which already used the "no principal means skip the check" rule.

Testing

• 2559/2559 tests pass, zero warnings under -warnaserror
• New AIUserAccessorTests covering the HTTP fallback, push/restore, null push, async continuation flow, and isolation between concurrent flows
• New handler tests covering "no caller skips authorization" and "anonymous caller is still evaluated"

Docs

• docs/core/tools.md — new IAIUserAccessor section documenting the resolution order and the null contract
• docs/changelog/v1.0.0.md — changelog entry