Fix HTTP task filter composition - #1722

Summary

Fixes #1720 by allowing ordinary CallToolFilters to compose with alternate-result filters used by Tasks and HTTP authorization.

• Matches the tool before either filter family runs.
• Runs alternate-result filters in registration order.
• Runs ordinary filters once, after alternate filters and before the tool.
• Runs ASP.NET Core authorization before task creation.
• Uses an independent request context and DI scope for background execution.
• Preserves exception conversion and tool lifecycle logging.

Filter order

Added in order                       Resulting structure
--------------                       -------------------
Alternate filter A             ┐     Alternate A(
Authorization filter           │       Authorization(
Tasks filter                   │         Tasks(
Alternate filter B             ├──>        Alternate B(
Ordinary filter C              │             Ordinary C(
Ordinary filter D              ┘               Ordinary D(
                                                    Tool))))))

For task-backed calls, everything inside Tasks(...) executes in the background. Alternate filters before Tasks execute before task creation; alternate filters after Tasks and all ordinary filters execute in the background.

Non-task calls run through the same pipeline synchronously.

An explicit CallToolWithAlternateHandler remains a replacement for the ordinary tool pipeline and cannot be combined with ordinary filters.

Fixes #1720.