Extract transaction state tracking logic (Phase 3.8, #5496)

New files:
- include/TransactionState.h: 2 pure function declarations
- lib/TransactionState.cpp: implementations

Functions:
- update_transaction_persistent_hostgroup(): mirrors session logic
  for locking/unlocking HG on transaction start/end
- is_transaction_timed_out(): checks if transaction exceeded max time

Logic identical for MySQL and PgSQL (shared via Base_Session).

include/TransactionState.h

@@ -0,0 +1,48 @@
+/**
+ * @file TransactionState.h
+ * @brief Pure transaction state tracking logic for unit testability.
+ *
+ * Extracted from MySQL_Session/PgSQL_Session transaction persistence
+ * logic. The decision is identical for both protocols.
+ *
+ * @see Phase 3.8 (GitHub issue #5496)
+ */
+
+#ifndef TRANSACTION_STATE_H
+#define TRANSACTION_STATE_H
+
+/**
+ * @brief Update transaction_persistent_hostgroup based on backend state.
+ *
+ * Mirrors the logic in MySQL_Session/PgSQL_Session (~lines 9276-9293):
+ * - When a transaction starts on a backend, lock to the current HG
+ * - When a transaction ends, unlock (-1)
+ *
+ * @param transaction_persistent         Whether transaction persistence is enabled.
+ * @param transaction_persistent_hostgroup Current persistent HG (-1 = none).
+ * @param current_hostgroup              HG where the query executed.
+ * @param backend_in_transaction         Whether the backend has an active transaction.
+ * @return Updated transaction_persistent_hostgroup value.
+ */
+int update_transaction_persistent_hostgroup(
+	bool transaction_persistent,
+	int transaction_persistent_hostgroup,
+	int current_hostgroup,
+	bool backend_in_transaction
+);
+
+/**
+ * @brief Check if a transaction has exceeded the maximum allowed time.
+ *
+ * @param transaction_started_at  Timestamp when transaction started (0 = none).
+ * @param current_time            Current timestamp.
+ * @param max_transaction_time_ms Maximum transaction time in milliseconds (0 = no limit).
+ * @return true if the transaction has exceeded the time limit.
+ */
+bool is_transaction_timed_out(
+	unsigned long long transaction_started_at,
+	unsigned long long current_time,
+	int max_transaction_time_ms
+);
+
+#endif // TRANSACTION_STATE_H

lib/Makefile

@@ -106,6 +106,7 @@ _OBJ_CXX := ProxySQL_GloVars.oo network.oo debug.oo configfile.oo Query_Cache.oo
 	PgSQL_PreparedStatement.oo PgSQL_Extended_Query_Message.oo \
 	pgsql_tokenizer.oo \
 	MonitorHealthDecision.oo \
+	TransactionState.oo \
 	proxy_sqlite3_symbols.oo
 
 # TSDB object files

lib/TransactionState.cpp

@@ -0,0 +1,55 @@
+/**
+ * @file TransactionState.cpp
+ * @brief Implementation of pure transaction state tracking.
+ *
+ * @see TransactionState.h
+ * @see Phase 3.8 (GitHub issue #5496)
+ */
+
+#include "TransactionState.h"
+
+int update_transaction_persistent_hostgroup(
+	bool transaction_persistent,
+	int transaction_persistent_hostgroup,
+	int current_hostgroup,
+	bool backend_in_transaction)
+{
+	if (!transaction_persistent) {
+		return -1;  // persistence disabled
+	}
+
+	if (transaction_persistent_hostgroup == -1) {
+		// Not currently locked — lock if transaction just started
+		if (backend_in_transaction) {
+			return current_hostgroup;
+		}
+	} else {
+		// Currently locked — unlock if transaction just ended
+		if (!backend_in_transaction) {
+			return -1;
+		}
+	}
+
+	return transaction_persistent_hostgroup;  // no change
+}
+
+bool is_transaction_timed_out(
+	unsigned long long transaction_started_at,
+	unsigned long long current_time,
+	int max_transaction_time_ms)
+{
+	if (transaction_started_at == 0) {
+		return false;  // no active transaction
+	}
+	if (max_transaction_time_ms <= 0) {
+		return false;  // no time limit
+	}
+
+	unsigned long long elapsed_ms = 0;
+	if (current_time > transaction_started_at) {
+		elapsed_ms = (current_time - transaction_started_at) / 1000;
+		// transaction_started_at and current_time are in microseconds
+	}
+
+	return (elapsed_ms > (unsigned long long)max_transaction_time_ms);
+}