Add %i prompt escape to indicate server read-only status

This patch introduces a new prompt escape `%i` for psql, which shows
whether the connected server is operating in read-only mode. It
expands to `read-only` if either the server is in hot standby mode
(`in_hot_standby = on`) or the session's default transaction mode is
read-only (`default_transaction_read_only = on`). Otherwise, it
displays `read/write`.

This is useful for distinguishing read-only sessions (e.g. connected
to a standby, or using a default read-only transaction mode) from
read/write ones at a glance, especially when working with multiple
connections in replicated or restricted environments.

Author: jimjonesbr

doc/src/sgml/ref/psql-ref.sgml

@@ -5044,6 +5044,20 @@ testdb=&gt; <userinput>INSERT INTO my_table VALUES (:'content');</userinput>
         </listitem>
       </varlistentry>
 
+      <varlistentry id="app-psql-prompting-i">
+        <term><literal>%i</literal></term>
+        <listitem>
+          <para>
+            Displays the session's read-only status as <literal>read-only</literal>
+            if the server is in hot standby (<literal>in_hot_standby</literal> is
+            <literal>on</literal>) or the default transaction mode is read-only
+            (<literal>default_transaction_read_only</literal> is <literal>on</literal>),
+            or <literal>read-write</literal> otherwise. Useful for identifying
+            sessions that cannot perform writes, such as in replication setups.
+          </para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry id="app-psql-prompting-x">
         <term><literal>%x</literal></term>
         <listitem>

src/bin/psql/prompt.c

@@ -43,6 +43,8 @@
  *			or a ! if session is not connected to a database;
  *		in prompt2 -, *, ', or ";
  *		in prompt3 nothing
+ * %i - displays "read-only" if in hot standby or default_transaction_read_only
+ *		is on, "read/write" otherwise.
  * %x - transaction status: empty, *, !, ? (unknown or no connection)
  * %l - The line number inside the current statement, starting from 1.
  * %? - the error code of the last query (not yet implemented)
@@ -247,7 +249,21 @@ get_prompt(promptStatus_t status, ConditionalStack cstack)
 							break;
 					}
 					break;
+				case 'i':
+					if (pset.db)
+					{
+						const char *hs = PQparameterStatus(pset.db, "in_hot_standby");
+						const char *ro = PQparameterStatus(pset.db, "default_transaction_read_only");
 
+						if ((hs && strcmp(hs, "on") == 0) ||
+							(ro && strcmp(ro, "on") == 0))
+							strlcpy(buf, "read-only", sizeof(buf));
+						else
+							strlcpy(buf, "read/write", sizeof(buf));
+					}
+					else
+						buf[0] = '\0';
+					break;
 				case 'x':
 					if (!pset.db)
 						buf[0] = '?';