New ``agent-framework-hosting`` package implementing ADR 0026 / SPEC-002:
the channel-neutral host that lets a single ``Agent`` (or ``Workflow``)
fan out across multiple wire protocols ("channels") behind one Starlette
ASGI app.

Surface (re-exported from ``agent_framework_hosting``):

- ``AgentFrameworkHost`` — wraps a hostable target, mounts channels onto
  an ASGI app, owns per-isolation-key ``AgentSession`` reuse, threads
  request context (``response_id`` / ``previous_response_id``) into
  context providers via an ``ExitStack`` of ``bind_request_context``
  calls, and exposes an opt-in Hypercorn ``serve()`` helper (extra
  ``[serve]``).
- ``Channel`` protocol + ``ChannelContribution`` — the surface a channel
  package implements (routes, lifespans, identity hooks, …).
- ``ChannelRequest`` / ``ChannelSession`` / ``ChannelIdentity`` /
  ``ChannelPush`` / ``ChannelCommand[Context]`` / ``ChannelRunHook`` /
  ``ChannelStreamTransformHook`` / ``DeliveryReport`` /
  ``HostedRunResult`` / ``ResponseTarget`` / ``ResponseTargetKind`` /
  ``apply_run_hook`` — channel-side dataclasses + helpers.
- ``IsolationKeys`` + ``ISOLATION_HEADER_USER`` / ``..._CHAT`` +
  ``get/set/reset_current_isolation_keys`` — the host's ASGI middleware
  reads the ``x-agent-{user,chat}-isolation-key`` headers off each
  inbound request and exposes them to the agent stack via a
  ``ContextVar`` so storage-side providers (e.g.
  ``FoundryHostedAgentHistoryProvider``) can apply per-tenant
  partitioning without channels having to forward anything.

Includes 45 unit tests covering the host, channel contributions,
isolation contextvar, and shared types. Registers the package in
``python/pyproject.toml`` ``[tool.uv.sources]`` and adds the matching
pyright ``executionEnvironments`` entry for tests.

Hypercorn is an optional dependency (``[serve]`` extra); the soft import
in ``serve()`` is annotated for pyright since it isn't on the default
install.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Source-code changes
- _suppress_already_consumed: narrow contract — RuntimeError now logs
  at WARNING with exc_info; non-RuntimeError still logs at exception().
  Docstring clarifies that any non-clean teardown is observable.
- _BoundResponseStream: add aclose() and route __await__ through
  get_final_response() so the binding is always released — fixes
  contextvar leak when channels abandon the stream or use the
  await-the-stream convenience.
- Lifespan: aggregate startup/shutdown callback errors; every callback
  runs, all failures are logged with their qualname, and the first
  error is re-raised so Starlette still aborts boot.
- _build_run_kwargs: switch session-cache write to dict.setdefault so
  concurrent racers cannot orphan a session if create_session ever
  yields.
- _deliver_response: introduce DeliveryReport.failed for push outages
  vs explicit "no link" drops; an outage no longer triggers an
  originating fallback so the channel can decide degraded behaviour.

Test additions
- tests/test_isolation.py (new): full coverage of IsolationKeys, the
  contextvar helpers, header constants, and end-to-end ASGI
  middleware lift / reset / passthrough.
- tests/test_host.py: TestBindRequestContext, TestBoundResponseStream
  (aclose / __await__ / __getattr__ forwarding / double-close
  idempotency), TestWrapInputListMessages (list[Message] LAST
  precedence), TestLifespanAggregation (startup + shutdown).
- tests/test_types.py: TestApplyRunHook (sync/async/None), and
  TestDeliveryReport (new failed field).
- Updated test_push_exception_marks_skipped ->
  test_push_exception_lands_in_failed_no_fallback to match the new
  delivery contract.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>