fix(toml): escape control characters so generated command files parse (#3341)

* fix(toml): escape control characters so generated command files parse

both toml renderers (TomlIntegration._render_toml_string for gemini/tabnine
and CommandRegistrar.render_toml_command for extension/preset commands) wrote
control characters raw into a multiline or basic string. toml forbids literal
control chars (U+0000-U+001F except tab/newline, and U+007F) in every string
form, and a bare CR that is not part of a CRLF pair, so a description or body
containing one produced a .toml file that fails to parse.

route any value with such a character to a fully-escaped basic string that
emits the leftover control chars as \uXXXX. added regression tests that
round-trip through tomllib.

* refactor(toml): centralize control-char escaping in one shared helper

the control-char detection and basic-string escaping added for both toml
renderers were copy-pasted into agents.py and integrations/base.py. move the
two functions into specify_cli/_toml_string.py and have both renderers
delegate to it, so the escaping rules can't drift apart later.

no behavior change; both renderers now reference the same implementation.

src/specify_cli/agents.py

@@ -16,6 +16,8 @@ from typing import Any, Dict, List, Optional
 import yaml
 
 from ._init_options import is_ai_skills_enabled, load_init_options
+from ._toml_string import escape_toml_basic as _escape_toml_basic
+from ._toml_string import has_illegal_toml_control as _has_illegal_toml_control
 from ._utils import relative_extension_path_violation
 
 
@@ -258,7 +260,12 @@ class CommandRegistrar:
         # ``C:\\Users\\...`` whose ``\\U`` reads as an invalid unicode escape) would
         # produce unparseable TOML — route those to the *literal* form ('''...'''),
         # which does not process escapes, or to the escaped basic string.
-        if '"""' not in body and "\\" not in body:
+        # Control characters (U+0000–U+001F except tab/newline, U+007F) and a bare
+        # CR are illegal in every TOML string form, so a body containing them must
+        # go to the escaped basic string regardless of which delimiters it uses.
+        if self._has_illegal_toml_control(body):
+            toml_lines.append(f"prompt = {self._render_basic_toml_string(body)}")
+        elif '"""' not in body and "\\" not in body:
             toml_lines.append('prompt = """')
             toml_lines.append(body)
             toml_lines.append('"""')
@@ -271,17 +278,11 @@ class CommandRegistrar:
 
         return "\n".join(toml_lines)
 
-    @staticmethod
-    def _render_basic_toml_string(value: str) -> str:
-        """Render *value* as a TOML basic string literal."""
-        escaped = (
-            value.replace("\\", "\\\\")
-            .replace('"', '\\"')
-            .replace("\n", "\\n")
-            .replace("\r", "\\r")
-            .replace("\t", "\\t")
-        )
-        return f'"{escaped}"'
+    # Control-char detection and basic-string escaping are shared with the
+    # gemini/tabnine renderer in ``specify_cli.integrations.base`` via
+    # ``specify_cli._toml_string`` so the two never drift apart.
+    _has_illegal_toml_control = staticmethod(_has_illegal_toml_control)
+    _render_basic_toml_string = staticmethod(_escape_toml_basic)
 
     def render_yaml_command(
         self,