test: lock gateway adapter capability surface (relay phase 0)

Behavioral regression harness locking the capability surface that the future
RelayAdapter must reproduce: the abstract-method set (connect/disconnect/send/
get_chat_info), message_len_fn default, supports_draft_streaming default, and
the stream_consumer MAX_MESSAGE_LENGTH attribute read. Passes on main before
any RelayAdapter exists.

Phase 0, Task 0.1 of the gateway-relay plan.

tests/gateway/test_relay_capability_surface.py

@@ -0,0 +1,99 @@
+"""Phase 0 regression harness for the relay/connector work.
+
+Locks the *behavioral contract* that the future ``RelayAdapter`` must reproduce:
+the gateway's ``stream_consumer`` and ``BasePlatformAdapter`` read per-platform
+capabilities through a small, stable surface. A relay adapter that exposes the
+same surface (``MAX_MESSAGE_LENGTH`` attribute, ``message_len_fn`` property,
+``supports_draft_streaming`` probe, and only the abstract methods) slots into
+the existing consumer with no consumer changes.
+
+These are deliberately *behavioral* (construct an adapter, drive the code,
+assert the observable outcome) rather than source-string snapshots, per the
+repo's "don't write change-detector tests" rule. They pass on ``main`` before
+any ``RelayAdapter`` exists — they describe the contract, not the relay.
+"""
+
+import inspect
+
+from gateway.config import Platform, PlatformConfig
+from gateway.platforms.base import BasePlatformAdapter
+
+
+class _MinAdapter(BasePlatformAdapter):
+    """Smallest concrete adapter: implements exactly the abstract methods."""
+
+    async def connect(self):  # pragma: no cover - not called
+        return True
+
+    async def disconnect(self):  # pragma: no cover - not called
+        return None
+
+    async def send(self, *args, **kwargs):  # pragma: no cover - not called
+        return None
+
+    async def get_chat_info(self, chat_id):  # pragma: no cover - not called
+        return {}
+
+
+def _make() -> BasePlatformAdapter:
+    return _MinAdapter(PlatformConfig(), Platform.LOCAL)
+
+
+def test_abstract_methods_are_the_known_set():
+    """The relay adapter must implement exactly this set of abstract methods.
+
+    Everything else on BasePlatformAdapter has a default, so ONE generic
+    RelayAdapter overriding the right subset is feasible without per-platform
+    gateway classes. If a new abstractmethod is added here, the relay design
+    (and the cross-repo contract) must be revisited — hence the lock.
+
+    NOTE: this is four methods, not three — ``get_chat_info`` is abstract too
+    (defined far below the connect/disconnect/send cluster in base.py). The
+    RelayAdapter must implement it (proxying a chat-info lookup to the
+    connector, or returning a descriptor-derived stub).
+    """
+    abstract = {
+        name
+        for name, member in inspect.getmembers(BasePlatformAdapter)
+        if getattr(member, "__isabstractmethod__", False)
+    }
+    assert abstract == {"connect", "disconnect", "send", "get_chat_info"}
+
+
+def test_message_len_fn_defaults_to_len():
+    """message_len_fn is the per-platform length-unit hook (Telegram overrides
+    it for UTF-16). The default is plain ``len``; the relay adapter will
+    override it from its negotiated descriptor's ``len_unit``."""
+    inst = _make()
+    assert inst.message_len_fn("hello") == 5
+
+
+def test_supports_draft_streaming_defaults_false():
+    """Draft streaming is opt-in per platform; the consumer falls back to the
+    edit-based path when False. The relay adapter flips this from its
+    descriptor's ``supports_draft_streaming`` flag."""
+    inst = _make()
+    assert inst.supports_draft_streaming() is False
+
+
+def test_stream_consumer_reads_max_message_length_by_attribute():
+    """The consumer resolves the per-platform char limit by reading the
+    adapter's ``MAX_MESSAGE_LENGTH`` attribute (defaulting to 4096 when
+    absent). The relay adapter exposes this as an attribute set from its
+    descriptor — so a relay adapter that sets the attribute is chunked
+    correctly with no consumer change.
+    """
+    from gateway import stream_consumer
+
+    class _NoLimit:
+        pass
+
+    class _WithLimit:
+        MAX_MESSAGE_LENGTH = 1234
+
+    assert getattr(_NoLimit(), "MAX_MESSAGE_LENGTH", 4096) == 4096
+    assert getattr(_WithLimit(), "MAX_MESSAGE_LENGTH", 4096) == 1234
+    # The consumer depends on BasePlatformAdapter for the message_len_fn
+    # isinstance guard (import-level contract the relay adapter satisfies by
+    # subclassing BasePlatformAdapter).
+    assert stream_consumer._BasePlatformAdapter is BasePlatformAdapter