tests: prepare test_compatibility.py for --timelines-onto-safekeepers (#12204)

## Problem
Compatibility tests may be run against a compatibility snapshot
generated with --timelines-onto-safekeepers=false. We need to start the
compute without a generation (or with 0 generation) if the timeline is
not storcon-managed, otherwise the compute will hang.

- Follow up on https://github.com/neondatabase/neon/pull/12203
- Relates to https://github.com/neondatabase/neon/pull/11712

## Summary of changes
- Handle compatibility snapshot generated with no
`--timelines-onot-safekeepers` properly

test_runner/regress/test_compatibility.py

@@ -19,6 +19,7 @@ from fixtures.neon_fixtures import (
     NeonEnvBuilder,
     PgBin,
     Safekeeper,
+    StorageControllerApiException,
     flush_ep_to_pageserver,
 )
 from fixtures.pageserver.http import PageserverApiException
@@ -301,7 +302,20 @@ def test_forward_compatibility(
 def check_neon_works(env: NeonEnv, test_output_dir: Path, sql_dump_path: Path, repo_dir: Path):
     ep = env.endpoints.create("main")
     ep_env = {"LD_LIBRARY_PATH": str(env.pg_distrib_dir / f"v{env.pg_version}/lib")}
-    ep.start(env=ep_env)
+
+    # If the compatibility snapshot was created with --timelines-onto-safekeepers=false,
+    # we should not pass safekeeper_generation to the endpoint because the compute
+    # will not be able to start.
+    # Zero generation is INVALID_GENERATION.
+    generation = 0
+    try:
+        res = env.storage_controller.timeline_locate(env.initial_tenant, env.initial_timeline)
+        generation = res["generation"]
+    except StorageControllerApiException as e:
+        if e.status_code != 404 or not re.search(r"Timeline .* not found", str(e)):
+            raise e
+
+    ep.start(env=ep_env, safekeeper_generation=generation)
 
     connstr = ep.connstr()
 
@@ -351,7 +365,7 @@ def check_neon_works(env: NeonEnv, test_output_dir: Path, sql_dump_path: Path, r
     )
 
     # Timeline exists again: restart the endpoint
-    ep.start(env=ep_env)
+    ep.start(env=ep_env, safekeeper_generation=generation)
 
     pg_bin.run_capture(
         ["pg_dumpall", f"--dbname={connstr}", f"--file={test_output_dir / 'dump-from-wal.sql'}"]