Add test to verify limit defaults to 100 for thread subscriptions extension

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Addresses MSC4308 discussion: the limit param should default to 100 when not specified.

The code already has this default (limit: StrictInt = 100 in synapse/types/rest/client/__init__.py), but no test explicitly verified this behavior.

Changes:

• Added test_limit_defaults_to_100 which syncs without specifying limit and confirms all subscriptions are returned

# Sync without specifying 'limit' - should default to 100
sync_body = {
    "extensions": {EXT_NAME: {"enabled": True}},  # no limit specified
}
response_body, _ = self.do_sync(sync_body, tok=user1_tok)

# All 5 subscriptions returned (default limit 100 > 5)
self.assertEqual(len(thread_subscriptions["subscribed"][room_id]), 5)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>MSC4308 Thread Subscriptions extension to Sliding Sync: limit request param in Sliding Sync extension should default to 100</issue_title>
<issue_description>matrix-org/matrix-spec-proposals#4308 (comment)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes MSC4308 Thread Subscriptions extension to Sliding Sync: limit request param in Sliding Sync extension should default to 100 #18942

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