Improve TOML serialization error message for unsupported types

[max-sixty]

Summary

• Replace bare .unwrap() in TOML serialization with an informative panic that shows the original toml_edit error and explains TOML's top-level struct/map requirement
• Add test for top-level sequence case (issue assert_toml_snapshot seems broken #879)
• Update existing unit struct test to match the improved error text

Closes #879. Thanks to @ip1981 for reporting.

Before:

called `Result::unwrap()` on an `Err` value: UnsupportedType(None)

After (top-level sequence):

TOML serialization failed: unsupported rust type. Note: TOML requires the
top-level value to be a struct or map. Use assert_json_snapshot! or
assert_yaml_snapshot! for other types.

After (unsupported inner type):

TOML serialization failed: unsupported Marker type. Note: TOML requires the
top-level value to be a struct or map. Use assert_json_snapshot! or
assert_yaml_snapshot! for other types.

Test plan

• New test_toml_top_level_sequence_unsupported test verifies the error message
• Existing test_toml_unit_struct_unsupported updated to match improved text
• All 34 TOML tests pass
• Full test suite passes

This was written by Claude Code on behalf of @max-sixty