storcon: validate new_sk_set before starting safekeeper migration (#12546)

## Problem
We don't validate the validity of the `new_sk_set` before starting the
migration. It is validated later, so the migration to an invalid
safekeeper set will fail anyway. But at this point we might already
commited an invalid `new_sk_set` to the database and there is no `abort`
command yet (I ran into this issue in neon_local and ruined the timeline
:)

- Part of https://github.com/neondatabase/neon/issues/11669

## Summary of changes
- Add safekeeper count and safekeeper duplication checks before starting
the migration
- Test that we validate the `new_sk_set` before starting the migration
- Add `force` option to the `TimelineSafekeeperMigrateRequest` to
disable not-mandatory checks

test_runner/regress/test_safekeeper_migration.py

@@ -2,6 +2,9 @@ from __future__ import annotations
 
 from typing import TYPE_CHECKING
 
+import pytest
+from fixtures.neon_fixtures import StorageControllerApiException
+
 if TYPE_CHECKING:
     from fixtures.neon_fixtures import NeonEnvBuilder
 
@@ -75,3 +78,38 @@ def test_safekeeper_migration_simple(neon_env_builder: NeonEnvBuilder):
     ep.start(safekeeper_generation=1, safekeepers=[3])
 
     assert ep.safe_psql("SELECT * FROM t") == [(i,) for i in range(1, 4)]
+
+
+def test_new_sk_set_validation(neon_env_builder: NeonEnvBuilder):
+    """
+    Test that safekeeper_migrate validates the new_sk_set before starting the migration.
+    """
+    neon_env_builder.num_safekeepers = 3
+    neon_env_builder.storage_controller_config = {
+        "timelines_onto_safekeepers": True,
+        "timeline_safekeeper_count": 2,
+    }
+    env = neon_env_builder.init_start()
+
+    def expect_fail(sk_set: list[int], match: str):
+        with pytest.raises(StorageControllerApiException, match=match):
+            env.storage_controller.migrate_safekeepers(
+                env.initial_tenant, env.initial_timeline, sk_set
+            )
+        # Check that we failed before commiting to the database.
+        mconf = env.storage_controller.timeline_locate(env.initial_tenant, env.initial_timeline)
+        assert mconf["generation"] == 1
+
+    expect_fail([], "safekeeper set is empty")
+    expect_fail([1], "must have at least 2 safekeepers")
+    expect_fail([1, 1], "duplicate safekeeper")
+    expect_fail([1, 100500], "does not exist")
+
+    mconf = env.storage_controller.timeline_locate(env.initial_tenant, env.initial_timeline)
+    sk_set = mconf["sk_set"]
+    assert len(sk_set) == 2
+
+    decom_sk = [sk.id for sk in env.safekeepers if sk.id not in sk_set][0]
+    env.storage_controller.safekeeper_scheduling_policy(decom_sk, "Decomissioned")
+
+    expect_fail([sk_set[0], decom_sk], "decomissioned")