Add chain-of-messages tests for response parser (#126)

The existing single-message parser tests can pass even if the parser
doesn't fully consume a message (e.g., missing a null terminator),
because there's no subsequent message whose parsing would be corrupted.
These chain tests concatenate multiple messages into a single buffer and
verify the parser correctly advances through each one.

Adds a property-based test with a generator covering all 26 parseable
message types (random sequences of 2-8 messages, verifying each parse
yields non-None and the buffer is fully consumed), plus 7 example-based
tests covering realistic protocol sequences with field-level assertions:
SimpleQueryResult, CopyOutSequence, EmptyQuerySequence, PrepareSequence,
CloseStatementSequence, SASLFullSequence, and RemainingTypes.

Resolves the TODO at the former line 6 of `_test_response_parser.pony`.

Author: SeanTAllen

postgres/_test.pony

@@ -120,6 +120,15 @@ actor \nodoc\ Main is TestList
     test(_TestResponseParserAuthenticationSASLFinalMessage)
     test(_TestResponseParserUnsupportedAuthenticationMessage)
     test(_TestResponseParserMultipleMessagesSASLFirst)
+    test(Property1UnitTest[Array[Array[U8] val] val](
+      _TestResponseParserMultipleMessagesChainProperty))
+    test(_TestResponseParserMultipleMessagesChainSimpleQueryResult)
+    test(_TestResponseParserMultipleMessagesChainCopyOutSequence)
+    test(_TestResponseParserMultipleMessagesChainEmptyQuerySequence)
+    test(_TestResponseParserMultipleMessagesChainPrepareSequence)
+    test(_TestResponseParserMultipleMessagesChainCloseStatementSequence)
+    test(_TestResponseParserMultipleMessagesChainSASLFullSequence)
+    test(_TestResponseParserMultipleMessagesChainRemainingTypes)
     test(_TestFrontendMessageSASLInitialResponse)
     test(_TestFrontendMessageSASLResponse)
     test(_TestScramSha256MessageBuilders)