Stop preset tests from waiting on the tail poll

AGENTS.md

@@ -39,6 +39,8 @@ Before touching a subsystem, read the relevant notes in `contributing/`: `ARCHIT
 - Speed up large runs with `-n auto` (pytest-xdist), e.g. `uv run pytest -n auto`.
 - Group tests for the same unit (function/class) using `Test*` classes that mirror unit's name.
 - Keep tests hermetic (network disabled except localhost per `[tool.pytest.ini_options]` in `pyproject.toml`); stub cloud calls with mocks.
+- Keep the suite fast. Never let a test wait on real time.
+- Machinery that costs hundreds of milliseconds per test (spawning a subprocess, starting a container, generating a key, real HTTP) has to earn its place by covering something cheaper tests cannot. Say so in the test when it does. On the other hand, losing black-box coverage or making the test hard to follow is worse than the milliseconds it costs.
 
 ## Commit & Pull Request Guidelines
 - Name branches `issue_{issue_num}_{title}` when the work tracks an issue (e.g. `issue_3959_replicated_alb_gateways`), and `pr_{title}` otherwise.

src/tests/_internal/cli/services/presets/conftest.py

@@ -0,0 +1,15 @@
+import pytest
+
+from dstack._internal.cli.services.presets.tail import _FileLineReader
+
+
+@pytest.fixture(autouse=True)
+def no_tail_poll_wait(monkeypatch: pytest.MonkeyPatch):
+    """
+    Stops the tailers from waiting between reads.
+
+    The agent writes its output to files rather than pipes, so reading it means polling,
+    and `_POLL_SECONDS` makes every run wait once after the agent has already exited.
+    Tests write the whole output up front, so there is nothing to wait for.
+    """
+    monkeypatch.setattr(_FileLineReader, "_POLL_SECONDS", 0)