Fix flaky conformance tests: duplicate test cases, server stdout, nonconforming

Summary:
Fix conformance test flakiness across round-trip and RPC test suites (24 tests).

1. TestGenerator.cpp: The empty_optional test case was generated inside the
   getInterestingValues() loop via addTestCase(), creating 30-50+ duplicate
   test cases per map type. Each duplicate sent a separate request to the hack
   conformance server, multiplying Queue Timeout failures. Moved the
   empty_optional case outside the loop so it is generated exactly once per
   type, matching the intended behavior.

2. conformance_server.py: Removed extra print('\n', flush=True) after port
   output. When the parent process closes the stdout pipe after reading the
   port, this extra write raised a BrokenPipeError causing the server to exit
   before it could handle requests.

3. rpc_server.py: Removed embedded '\n' from the port print statement
   (print(f"{addr.port}\n") -> print(addr.port)). The extra newline caused
   the parent process to read an empty second line as the port, failing to
   connect to the server.

4. nonconforming.txt: Added rust SinkDeclaredException test to the
   nonconforming list, matching the existing SinkUndeclaredException entry.
   This is a known runtime exception encoding non-conformance (T218969298).

Reviewed By: evanjzou

Differential Revision: D94987645

fbshipit-source-id: 1b93f4d909a77bf2634ac1333370452ef6a0c2d8

Author: sazonovkirill

third-party/thrift/src/thrift/conformance/data/TestGenerator.cpp

@@ -77,14 +77,22 @@ Test createRoundTripTest(
     // Test case #3: Optional field
     addStruct(mod_set<FieldModifier::Optional>{}, "Optional.");
 
-    addTestCase(
+    // Test case #4: Terse field
+    addStruct(mod_set<FieldModifier::Terse>{}, "Terse.");
+  }
+
+  // Test case: Optional field with empty value (generated once, not per value)
+  {
+    RoundTripTestCase roundTrip;
+    roundTrip.request()->value() = registry.store(
         typename struct_ByFieldType<
             ElementTag,
             mod_set<FieldModifier::Optional>>::type{},
-        fmt::format("testset.Optional.{}/empty_optional", typeName));
-
-    // Test case #4: Terse field
-    addStruct(mod_set<FieldModifier::Terse>{}, "Terse.");
+        protocol);
+    auto& emptyOptTestCase = test.testCases()->emplace_back();
+    emptyOptTestCase.name() =
+        fmt::format("testset.Optional.{}/empty_optional", typeName);
+    emptyOptTestCase.test()->roundTrip() = roundTrip;
   }
 
   return test;

third-party/thrift/src/thrift/conformance/python/conformance_server.py

@@ -51,7 +51,6 @@ async def main():
     serve_task = asyncio.create_task(server.serve())
     addr = await server.get_address()
     print(addr.port, flush=True)
-    print("\n", flush=True)
     try:
         await serve_task
     finally:

third-party/thrift/src/thrift/conformance/python/rpc_server.py

@@ -78,7 +78,7 @@ async def main():
     for signal in [SIGINT, SIGTERM]:
         loop.add_signal_handler(signal, server.stop)
     addr = await server.get_address()
-    print(f"{addr.port}\n", flush=True)
+    print(addr.port, flush=True)
     await serve_task