Python: add agent-framework-hosting-entra identity-link helpers (#5644)

* feat(hosting-entra): add Entra (Azure AD) identity-linking channel

New ``agent-framework-hosting-entra`` package implementing a Microsoft
Entra OAuth-based identity-linking channel for the Hosting framework.
Mounts a small set of routes (``/entra/login``, ``/entra/callback``,
``/entra/whoami``) that walk a user through an Entra/Azure AD
authorization-code flow and stick the resulting verified identity
(``oid`` / ``email`` / ``tid``) onto the host's identity table so
later requests on any other channel (Responses, Telegram, …) can be
linked to the same user.

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

- ``EntraChannel`` -- concrete ``Channel`` implementation. Owns the
  three Starlette routes, signs/verifies short-lived ``state`` tokens
  to bind the round-trip to the originating channel, exchanges the
  authorization code for an ID token via MSAL, and writes the
  verified identity into the host's identity store via the standard
  ``ChannelIdentity`` plumbing so cross-channel push (e.g. send a
  Telegram message to the user who completed the link from
  Responses) works without the channels having to coordinate
  directly.
- 14 unit tests covering route wiring, ``state`` issue / verify,
  callback exchange happy + failure paths, and identity-store write.

Registers the package in ``python/pyproject.toml``
``[tool.uv.sources]`` and adds the matching pyright
``executionEnvironments`` entry. Stacks on PR-2 (Hosting core);
independent of PR-3 / PR-4 / PR-6.

The cross-channel sample (``local_identity_link/``) that demonstrates
this end-to-end alongside Responses + Telegram lands in PR-8 (samples).

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

* fix(hosting-entra): close IDOR + reflected-XSS + open-redirect on the OAuth flow

Three SECURITY-CRITICAL fixes flagged in round-2 review.

1. IDOR on /auth/start (3198518308). Without authentication the
   endpoint accepted (channel, channel_id) from the query string and
   bound *whoever signed in* to that pair. An attacker could bind
   their own Entra oid to a victim's per-channel id (e.g.
   `telegram:<victim_chat_id>`), redirecting all of the victim's
   future inbound traffic to the attacker's isolation key.

   Fix: introduce link_token_secret + mint_start_url(channel, id, ...).
   When set, /auth/start requires `exp` + `sig` (HMAC-SHA256 over
   `channel|channel_id|expires_at`) before issuing the redirect.
   Channels that hand out start URLs (a Telegram /link command after
   verifying the inbound webhook signature) call mint_start_url so
   the token proves the (channel, id) pair was authorised by the
   channel that owns the surface. Unsigned mode is opt-in and logs a
   loud WARNING at startup *and* on every accepted request.

2. Reflected XSS on /auth/callback (3198520256, 3198527896). `error`,
   `error_description`, channel_key (from the unauthenticated /start
   query), and `upn` (from a Graph response) flowed straight into the
   text/html response body unescaped. With the IDOR above, an
   attacker could stash `<script>` payloads in `channel` or `id` and
   serve them from the auth host's origin (full XSS on the auth
   surface — cookies/storage of anything else mounted there).

   Fix: html.escape() every value before HTML output.

3. Open redirect on `return_to` (3198524746). Accepted any URL.

   Fix: `_validate_return_to` allows only relative paths starting
   with `/` (and not `//`) or absolute URLs whose host equals the
   configured `public_base_url` host. Validated at /start mint time
   AND defensively re-validated at /callback before redirect.

12 new tests cover signed-token rejection (missing/forged/expired),
mint helper requirements, startup warning visibility, XSS escaping
on both error and success paths, and the open-redirect allowlist
(external rejected, relative accepted, same-origin accepted,
protocol-relative `//evil.example/` rejected).

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

* test(hosting): drop redundant @pytest.mark.asyncio decorators

asyncio_mode = "auto" is configured in pyproject.toml across the
hosting packages, so individual @pytest.mark.asyncio decorators are
unnecessary.

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

---------

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

python/uv.lock

@@ -52,6 +52,7 @@ members = [
     "agent-framework-hosting-invocations",
     "agent-framework-hosting-telegram",
     "agent-framework-hosting-activity-protocol",
+    "agent-framework-hosting-entra",
     "agent-framework-hyperlight",
     "agent-framework-lab",
     "agent-framework-mem0",
@@ -656,6 +657,18 @@ dependencies = [
     { name = "httpx", marker = "sys_platform == 'darwin' or sys_platform == 'linux' or sys_platform == 'win32'" },
 ]
 
+[[package]]
+name = "agent-framework-hosting-entra"
+version = "1.0.0a260424"
+source = { editable = "packages/hosting-entra" }
+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 = "cryptography", marker = "sys_platform == 'darwin' or sys_platform == 'linux' or sys_platform == 'win32'" },
+    { name = "httpx", marker = "sys_platform == 'darwin' or sys_platform == 'linux' or sys_platform == 'win32'" },
+    { name = "msal", marker = "sys_platform == 'darwin' or sys_platform == 'linux' or sys_platform == 'win32'" },
+]
+
 [[package]]
 name = "agent-framework-hosting-responses"
 version = "1.0.0a260424"
@@ -665,6 +678,7 @@ dependencies = [
     { 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 = [
     { name = "agent-framework-core", editable = "packages/core" },
@@ -692,6 +706,9 @@ dependencies = [
     { name = "agent-framework-hosting", marker = "sys_platform == 'darwin' or sys_platform == 'linux' or sys_platform == 'win32'" },
     { name = "azure-identity", specifier = ">=1.20,<2" },
     { name = "httpx", specifier = ">=0.27,<1" },
+    { name = "cryptography", specifier = ">=42" },
+    { name = "httpx", specifier = ">=0.27,<1" },
+    { name = "msal", specifier = ">=1.28,<2" },
 ]
 
 [[package]]