feat: add stream context to middleware and handlers

[dozyio]

Description

Adds per-stream context that can be shared between stream middleware and the final stream handler.

This allows middleware to attach typed values during stream setup and lets the handler read them without mutating the stream or connection objects.

Includes regression coverage for:

• passing context from incoming middleware to the handler
• creating a fresh context for each incoming stream

Notes & open questions

Requires #3534

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works