fix(generator): emit .msg syntax in _fields_and_field_types for bounded types

_fields_and_field_types / get_fields_and_field_types() is documented as a
Python introspection API for message field types. However it was emitting
OMG IDL 4.2 syntax for bounded strings and sequences:

    sequence<string<256>, 32>   (IDL)

instead of the canonical ROS 2 .msg syntax:

    string<=256[<=32]           (.msg)

This breaks any consumer that passes the output directly to the MCAP
ros2msg schema encoder (e.g. mcap_utils _build_ros2_schema()), because
the ros2msg schema encoding requires .msg syntax per the MCAP registry
spec (https://mcap.dev/spec/registry#ros2msg). Foxglove and other
ros2msg-aware tools reject the IDL form.

Changes:
- Remove IDL-style sequence<> prefix/suffix generation in the
  _fields_and_field_types dict template.
- Emit bounded string bound with <= (string<=N) instead of <N>.
- Emit bounded sequence bound as [<=N], unbounded sequence as [],
  fixed-size array as [N] -- all matching .msg syntax.
- Update test_interfaces.py expected values accordingly.

The SLOT_TYPES tuple (rosidl_parser.definition objects) is unchanged
and remains the correct API for programmatic type inspection.

Author: vik-shah-ai

rosidl_generator_py/resource/_msg.py.em

@@ -268,11 +268,7 @@ if isinstance(type_, AbstractNestedType):
     type_ = type_.value_type
 }@
         '@(member.name)': '@
-@# the prefix for nested types
-@[  if isinstance(member.type, AbstractSequence)]@
-sequence<@
-@[  end if]@
-@# the typename of the non-nested type or the nested basetype
+@# the typename of the non-nested type or the nested basetype (.msg syntax)
 @[  if isinstance(type_, BasicType)]@
 @(type_.typename)@
 @[  elif isinstance(type_, AbstractGenericString)]@
@@ -282,17 +278,16 @@ w@
 @[    end if]@
 string@
 @[    if type_.has_maximum_size()]@
-<@(type_.maximum_size)>@
+<=@(type_.maximum_size)@
 @[    end if]@
 @[  elif isinstance(type_, NamespacedType)]@
 @('/'.join([type_.namespaces[0], type_.name]))@
 @[  end if]@
-@# the suffix for nested types
-@[  if isinstance(member.type, AbstractSequence)]@
-@[    if isinstance(member.type, BoundedSequence)]@
-, @(member.type.maximum_size)@
-@[    end if]@
->@
+@# array/sequence suffix in .msg syntax
+@[  if isinstance(member.type, BoundedSequence)]@
+[<=@(member.type.maximum_size)]@
+@[  elif isinstance(member.type, UnboundedSequence)]@
+[]@
 @[  elif isinstance(member.type, Array)]@
 [@(member.type.size)]@
 @[  end if]@

rosidl_generator_py/test/test_interfaces.py

@@ -939,17 +939,17 @@ def test_string_slot_attributes() -> None:
     assert hasattr(msg, '__slots__')
     string_slot_types_dict = getattr(msg, 'get_fields_and_field_types')()
     expected_string_slot_types_dict = {
-        'ub_string_static_array_value': 'string<5>[3]',
-        'ub_string_ub_array_value': 'sequence<string<5>, 10>',
-        'ub_string_dynamic_array_value': 'sequence<string<5>>',
-        'string_dynamic_array_value': 'sequence<string>',
+        'ub_string_static_array_value': 'string<=5[3]',
+        'ub_string_ub_array_value': 'string<=5[<=10]',
+        'ub_string_dynamic_array_value': 'string<=5[]',
+        'string_dynamic_array_value': 'string[]',
         'string_static_array_value': 'string[3]',
-        'string_bounded_array_value': 'sequence<string, 10>',
-        'def_string_dynamic_array_value': 'sequence<string>',
+        'string_bounded_array_value': 'string[<=10]',
+        'def_string_dynamic_array_value': 'string[]',
         'def_string_static_array_value': 'string[3]',
-        'def_string_bounded_array_value': 'sequence<string, 10>',
-        'def_various_quotes': 'sequence<string>',
-        'def_various_commas': 'sequence<string>',
+        'def_string_bounded_array_value': 'string[<=10]',
+        'def_various_quotes': 'string[]',
+        'def_various_commas': 'string[]',
     }
 
     assert len(string_slot_types_dict) == len(expected_string_slot_types_dict)
@@ -1012,7 +1012,7 @@ def test_builtin_sequence_slot_attributes() -> None:
     assert hasattr(msg, '__slots__')
     builtin_sequence_slot_types_dict = getattr(msg, 'get_fields_and_field_types')()
     expected_builtin_sequence_slot_types_dict = {
-        'char_sequence_unbounded': 'sequence<char>',
+        'char_sequence_unbounded': 'char[]',
     }
 
     assert len(builtin_sequence_slot_types_dict) == len(expected_builtin_sequence_slot_types_dict)