storcon: Fix migration for Attached(0) tenants (#12256)

## Problem

`Attached(0)` tenant migrations can get stuck if the heatmap file has
not been uploaded.

## Summary of Changes

- Added a test to reproduce the issue.
- Introduced a `kick_secondary_downloads` config flag:
  - Enabled in testing environments.
  - Disabled in production (and in the new test).
- Updated `Attached(0)` locations to consider the number of secondaries
in their intent when deciding whether to download the heatmap.

test_runner/fixtures/neon_fixtures.py

@@ -453,6 +453,7 @@ class NeonEnvBuilder:
         pageserver_get_vectored_concurrent_io: str | None = None,
         pageserver_tracing_config: PageserverTracingConfig | None = None,
         pageserver_import_config: PageserverImportConfig | None = None,
+        storcon_kick_secondary_downloads: bool | None = None,
     ):
         self.repo_dir = repo_dir
         self.rust_log_override = rust_log_override
@@ -514,6 +515,8 @@ class NeonEnvBuilder:
         self.pageserver_tracing_config = pageserver_tracing_config
         self.pageserver_import_config = pageserver_import_config
 
+        self.storcon_kick_secondary_downloads = storcon_kick_secondary_downloads
+
         self.pageserver_default_tenant_config_compaction_algorithm: dict[str, Any] | None = (
             pageserver_default_tenant_config_compaction_algorithm
         )
@@ -1221,6 +1224,14 @@ class NeonEnv:
             else:
                 cfg["storage_controller"] = {"use_local_compute_notifications": False}
 
+        if config.storcon_kick_secondary_downloads is not None:
+            # Configure whether storage controller should actively kick off secondary downloads
+            if "storage_controller" not in cfg:
+                cfg["storage_controller"] = {}
+            cfg["storage_controller"]["kick_secondary_downloads"] = (
+                config.storcon_kick_secondary_downloads
+            )
+
         # Create config for pageserver
         http_auth_type = "NeonJWT" if config.auth_enabled else "Trust"
         pg_auth_type = "NeonJWT" if config.auth_enabled else "Trust"