fix(ometa): address PR review on SSE client

Three follow-ups from Copilot's review on PR open-metadata#28293:

1. Forward `ClientConfig.cert` to the SSE request so mTLS / client-cert
   setups work for SSE the same way they do for the REST client. Without
   this, deployments that authenticate the REST path via mutual TLS would
   silently fail on `/agents/dynamic/run` and similar SSE endpoints.

2. Document `data` and the new `timeout` parameter in the `stream()`
   docstring, including the SSE-specific note that the default of `None`
   disables timeouts because SSE streams can have long idle periods.

3. Migrate unit tests off `httpx` mocks:
     - `tests/unit/metadata/ingestion/ometa/test_sse_client.py` and
       `tests/unit/test_user_agent.py` now mock `requests.Session.request`
       instead of `httpx.Client.stream`.
     - `MockHTTPXClient` is replaced by `MockRequestsSession` that
       accepts the SDK's full kwargs set (method/url/headers/json/params/
       stream/timeout/verify/allow_redirects/cookies/cert).
     - `MockSSEResponse.iter_lines` now accepts `decode_unicode` to
       match the call shape of the new code.
     - HTTP error / connection error fixtures use `requests.exceptions`.

Not adopting Copilot's suggestion to fall back to `self.config.timeout`
when the stream() arg is `None`: the REST client's default is `(10, 300)`,
which would silently abort SSE streams that have legitimate idle gaps
longer than 5 minutes (LLM "thinking" pauses, sparse update events).
Keeping the SSE default at `None` (no timeout) preserves prior behavior
and matches the working-everywhere baseline; callers can opt in.

All 56 tests in the two updated files pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: pmbrull

ingestion/src/metadata/ingestion/ometa/sse_client.py

@@ -48,6 +48,13 @@ def stream(
         Args:
             method (str): The HTTP method to use.
             path (str): The path to the SSE stream.
+            data (dict | None): Request body sent as JSON for non-GET methods, or as
+                query parameters for GET. Defaults to None (no body / no params).
+            timeout (float | tuple[float, float] | None): Per-call timeout passed to
+                ``requests``. ``None`` (the default) disables timeouts, which matches
+                SSE semantics where streams can have long idle periods between events.
+                Pass a single float to set both connect and read timeouts, or a
+                ``(connect, read)`` tuple to set them independently.
 
         Returns:
             Generator[Any, Any, None]: A generator of events.
@@ -106,6 +113,7 @@ def stream(
                         self.config.allow_redirects if self.config.allow_redirects is not None else True
                     ),
                     "cookies": self.config.cookies,
+                    "cert": self.config.cert,
                 }
                 with requests.Session() as session, session.request(**request_kwargs) as response:
                     response.raise_for_status()