Fix pageserver_try_receive (#11096)

## Problem

See https://neondb.slack.com/archives/C04DGM6SMTM/p1741176713523469

The problem is that this function is using `PQgetCopyData(shard->conn,
&resp_buff.data, 1 /* async = true */)`
to try to fetch next message. But this function returns 0 if the whole
message is not present in the buffer.
And input buffer may contain only part of message so result is not
fetched.

## Summary of changes

Use `PQisBusy` + `WaitEventSetWait` to check if data is available and
`PQgetCopyData(shard->conn, &resp_buff.data, 0)` to read whole message
in this case.

---------

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

test_runner/regress/test_lfc_working_set_approximation.py

@@ -19,10 +19,7 @@ def test_lfc_working_set_approximation(neon_simple_env: NeonEnv):
     log.info("Creating endpoint with 1MB shared_buffers and 64 MB LFC")
     endpoint = env.endpoints.create_start(
         "main",
-        config_lines=[
-            "neon.max_file_cache_size='128MB'",
-            "neon.file_cache_size_limit='64MB'",
-        ],
+        config_lines=["neon.max_file_cache_size='128MB'", "neon.file_cache_size_limit='64MB'"],
     )
 
     cur = endpoint.connect().cursor()
@@ -116,4 +113,4 @@ def test_sliding_working_set_approximation(neon_simple_env: NeonEnv):
     log.info(f"Table size {size} blocks")
 
     assert estimation_1k >= 20 and estimation_1k <= 40
-    assert estimation_10k >= 200 and estimation_10k <= 400
+    assert estimation_10k >= 200 and estimation_10k <= 440