docs: fill migration-guide gaps found by automated v1-to-v2 migration runs

A batch of automated v1-to-v2 migrations over the SDK's example
directories plus several real-world consumers surfaced recurring gaps in
docs/migration.md - places where migrating code had to fall back to
reading SDK source. This addresses the highest-frequency ones:

- Add "what is unchanged" inventories for MCPServer, the lowlevel
  Server serving scaffolding, and the auth surface (the single most
  common gap: the guide listed what changed but never confirmed what
  survived).
- Document server-initiated requests raising NoBackChannelError on
  2026-07-28 connections (including the default in-process
  Client(server)), with cross-links from the mode, testing, tasks, and
  deprecation sections.
- State the real streamable_http_client signature, the per-parameter
  porting map for the removed streamablehttp_client kwargs, and
  sse_client's retained parameters; note that mcp no longer installs
  httpx and that the auth providers now subclass httpx2.Auth.
- Fix the mounted-app example to include the required lifespan, and
  spell out which transport parameters live on run() vs the app
  factories.
- Document the client_secret_post client_id fix, JSONRPCError.id
  becoming RequestId | None, and MCPDeprecationWarning behavior under
  warnings-as-errors.
- Add porting guidance for the removed experimental Tasks API, the
  ProgressContext delta-vs-absolute semantics, and RootModel-union
  consequences for isinstance and .root access.
- Note the Client vs ClientSession pagination shapes, the per-request
  ServerSession identity change, and the current tool-only scope of the
  parametrized Context[X] annotation.

No-Verification-Needed: doc-only change

Author: maxisbey