Python: add agent-framework-hosting-invocations channel (#5640)

* feat(hosting-invocations): add Invocations channel package

New ``agent-framework-hosting-invocations`` package implementing the
"Invocations" HTTP channel for the Hosting framework -- a lightweight
JSON-over-HTTP shape (``POST /invocations``) for callers that want a
single request/response without committing to the full OpenAI Responses
envelope. Mounts onto an ``AgentFrameworkHost`` like any other channel.

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

- ``InvocationsChannel`` -- concrete ``Channel`` implementation. Owns
  the Starlette route, parses inbound JSON into a ``ChannelRequest``
  (``input`` / ``session`` / ``metadata`` / ``options``), runs the
  optional ``ChannelRunHook``, calls back into the ``ChannelContext``
  to invoke the agent target, and returns a flat JSON envelope (or an
  SSE stream when ``stream=true``).
- 8 unit tests covering route wiring, isolation-key passthrough, hook
  composition, sync vs streaming paths, and ack-only behaviour for
  non-originating ``DeliveryReport``s.

Registers the package in ``python/pyproject.toml`` ``[tool.uv.sources]``
and adds the matching pyright ``executionEnvironments`` entry.

Independent of PR-3 (Responses); both depend only on PR-2 (Hosting
core).

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

* review: address PR-4 round 2 feedback

- expand `_stream` docstring to call out the HTTP-200 + `event: error`
  SSE contract (status committed before generator runs; hard failures
  surface as the first SSE frame, not an HTTP code)
- split chunked text on full-line terminators via `splitlines()` so
  embedded `\r` / `\r\n` no longer leak into `data:` framing on the
  wire, breaking EventSource consumers
- on `get_final_response()` failure, emit `event: error` instead of
  silently swallowing — finalize is what triggers
  history-provider persistence on the agent side, so a 5xx /
  disk-full / context-provider error must reach the client
- add tests covering `stream_transform_hook` (rewrite, drop, async),
  CRLF-in-chunk framing, and the finalize-error → no-`[DONE]` contract

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

* docs(hosting-invocations): rename stale ChatMessage docstring reference to Message

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

* fix(hosting-invocations): adapt to hosted run result wrapper

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

* feat(hosting-invocations): add response hooks

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

---------

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

python/uv.lock

@@ -49,6 +49,7 @@ members = [
     "agent-framework-github-copilot",
     "agent-framework-hosting",
     "agent-framework-hosting-responses",
+    "agent-framework-hosting-invocations",
     "agent-framework-hyperlight",
     "agent-framework-lab",
     "agent-framework-mem0",
@@ -651,7 +652,6 @@ dependencies = [
     { name = "agent-framework-core", marker = "sys_platform == 'darwin' or sys_platform == 'linux' or sys_platform == 'win32'" },
     { name = "agent-framework-hosting", marker = "sys_platform == 'darwin' or sys_platform == 'linux' or sys_platform == 'win32'" },
     { name = "openai", marker = "sys_platform == 'darwin' or sys_platform == 'linux' or sys_platform == 'win32'" },
-]
 
 [package.metadata]
 requires-dist = [
@@ -660,6 +660,15 @@ requires-dist = [
     { name = "openai", specifier = ">=1.99.0,<3" },
 ]
 
+[[package]]
+name = "agent-framework-hosting-invocations"
+version = "1.0.0a260424"
+source = { editable = "packages/hosting-invocations" }
+dependencies = [
+    { name = "agent-framework-core", marker = "sys_platform == 'darwin' or sys_platform == 'linux' or sys_platform == 'win32'" },
+    { name = "agent-framework-hosting", marker = "sys_platform == 'darwin' or sys_platform == 'linux' or sys_platform == 'win32'" },
+]
+
 [[package]]
 name = "agent-framework-hyperlight"
 version = "1.0.0b260521"