Cover activity log endpoints in org integration suite

[sdairs]

Summary

activity_get_list and activity_get have no live coverage.

Scope

• Place this phase after the custom-roles phase so the activity log has a known recent event to assert on (role create + delete).
• activity_get_list — fetch recent activity, assert non-empty.
• Find an activity entry attributable to this test run (matching the synthetic role name in the entry's metadata if available, or just an entry within the last few minutes for the current org).
• activity_get for that entry — assert fields populated.

Where this lands

• crates/clickhouse-cloud-api/tests/integration_org_test.rs (new phase, after roles)

Notes

• Activity log writes are eventually consistent — poll for the expected entry rather than asserting on the first read.
• If activity entries don't carry enough metadata to attribute precisely, fall back to "an entry with timestamp within the test window" as the assertion.

Parent: #151