bash: fix multiline PS1 with command substitutions (#11369)

Only replace the \n prompt escape when inserting secondary prompt marks,
not literal newlines `($'\n')`. Literal newlines may appear inside
`$(...)` or `...` command substitutions, and inserting escape sequences
there breaks the shell syntax. For example:

      PS1='$(if [ $? -eq 0 ]; then echo -e "P";
                    else echo -e "F";
                    fi) $ '

The literal newlines between the if/else/fi are part of the shell syntax
inside the command substitution. The previous code replaced all literal
newlines in PS1 with newline + OSC 133 escape sequences, which injected
terminal escapes into the middle of the command substitution and caused
bash to report a syntax error when evaluating it.

The \n prompt escape is PS1-specific and safe to replace globally. This
means prompts using literal newlines for line breaks (rather than \n)
won't get per-line secondary marks, but this is the conventional form
and avoids the need for complex shell parsing.

Fixes: #11267

src/shell-integration/bash/ghostty.bash

@@ -201,14 +201,16 @@ function __ghostty_precmd() {
     PS1='\[\e]133;A;redraw=last;cl=line;aid='"$BASHPID"'\a\]'$PS1'\[\e]133;B\a\]'
     PS2='\[\e]133;A;k=s\a\]'$PS2'\[\e]133;B\a\]'
 
-    # Bash doesn't redraw the leading lines in a multiline prompt so
-    # we mark the start of each line (after each newline) as a secondary
-    # prompt. This correctly handles multiline prompts by setting the first
-    # to primary and the subsequent lines to secondary.
-    if [[ "${PS1}" == *"\n"* || "${PS1}" == *$'\n'* ]]; then
-      builtin local __ghostty_mark=$'\\[\\e]133;A;k=s\\a\\]'
-      PS1="${PS1//$'\n'/$'\n'$__ghostty_mark}"
-      PS1="${PS1//\\n/\\n$__ghostty_mark}"
+    # Bash doesn't redraw the leading lines in a multiline prompt so we mark
+    # the start of each line (after each newline) as a secondary prompt. This
+    # correctly handles multiline prompts by setting the first to primary and
+    # the subsequent lines to secondary.
+    #
+    # We only replace the \n prompt escape, not literal newlines ($'\n'),
+    # because literal newlines may appear inside $(...) command substitutions
+    # where inserting escape sequences would break shell syntax.
+    if [[ "$PS1" == *"\n"* ]]; then
+      PS1="${PS1//\\n/\\n$'\\[\\e]133;A;k=s\\a\\]'}"
     fi
 
     # Cursor