test(tls): drain full stdout for the TLS-fallback assertion (flake fix)

test_tls_startup_failure_fallback_to_http read only the first 2000 bytes of
server stdout to assert 'TLS setup failed'. When the test interpreter lacks
optional deps (requests/websockets), startup emits a burst of plugin/tool
import warnings + the startup-config banner BEFORE the TLS line, pushing the
marker past the 2000-byte window -> spurious failure. Drain all
currently-available stdout (5s deadline, short-circuit on the marker) instead
of a single fixed-size read. Verified failing->passing in a fresh venv without
the optional deps. Test-only; distinct from the agent-package-isolation flake
cause already fixed in the same test.

CHANGELOG.md

@@ -19,6 +19,8 @@
 
 ### Internal
 
+- **Hardened `test_tls_support::test_tls_startup_failure_fallback_to_http` against stdout-preamble noise.** The test asserted the server prints "TLS setup failed" by reading only the first 2000 bytes of its stdout. When the test interpreter lacks optional deps (`requests`/`websockets`), startup emits a burst of plugin/tool import warnings plus the startup-config banner *before* the TLS line, pushing the marker past that fixed window → spurious failure (distinct from the agent-package-isolation cause fixed separately below). It now drains all currently-available stdout (bounded by a 5s deadline, short-circuiting once the marker is seen) instead of a single fixed-size read. Verified failing→passing in a fresh venv without the optional deps. Test-only.
+
 - **Fixed the last 2 chronic local full-suite test-isolation flakes.** `test_tls_support::test_tls_startup_failure_fallback_to_http` and `test_v050259_sessiondb_fd_leak::test_session_db_close_is_idempotent` failed only in a full local suite run (they skip on CI, where the agent package isn't co-located). Root cause was the same "simulate agent package unavailable" antipattern shipped-fixed for the profile cluster: `test_custom_provider_prefix_collisions` clobbered `sys.modules['agent']` at collection time (never restored), and several helpers emptied real package `__path__` lists in place. Fixed at the source — the collection-time shim is now guarded on `importlib.util.find_spec` (only installed when the real package is genuinely absent), the in-place `__path__` mutations use `monkeypatch.setattr(..., raising=False)`, and `test_issue1574`'s fake-agent helper restores the env/`sys.path` it mutates. Full local suite now 0 failed (was 2); `test_title_aux_routing` unaffected. Test-only.
 
 ### Fixed

tests/test_tls_support.py

@@ -295,14 +295,29 @@ class TestTLSEndToEnd(unittest.TestCase):
         self._proc = _start_and_wait(
             use_ssl=False, cert="/nonexistent/cert.pem", key="/nonexistent/key.pem",
         )
-        # Confirm TLS warning was printed
+        # Confirm the TLS warning was printed. Drain ALL currently-available
-        import fcntl
+        # stdout, not just the first N bytes: startup can emit an unbounded
+        # amount of unrelated preamble first (optional-dep plugin/tool import
+        # warnings when the test venv lacks `requests`/`websockets`, the startup
+        # config banner, the state-dir hint, etc.), which can push the
+        # "TLS setup failed" line well past any fixed-size single read and make
+        # this test flaky depending on how noisy the environment is.
         os.set_blocking(self._proc.stdout.fileno(), False)
         output = ""
-        try:
+        deadline = time.time() + 5
-            output = self._proc.stdout.read(2000) or ""
+        while time.time() < deadline:
-        except BlockingIOError:
+            try:
-            output = ""
+                chunk = self._proc.stdout.read(65536)
+            except BlockingIOError:
+                chunk = None
+            if chunk:
+                output += chunk
+                if "TLS setup failed" in output:
+                    break
+            else:
+                if "TLS setup failed" in output:
+                    break
+                time.sleep(0.05)
         self.assertIn("TLS setup failed", output)