fktn-k / fkYAML / 28809106979

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Fix serialization of strings that are invalid plain scalars (#523)

* Fix serialization of strings that are invalid plain scalars

Quote string scalars that cannot be safely emitted in YAML plain style,
including values starting with directive/reserved/flow indicator characters
such as `%`, `@`, `!`, `&`, and `{`.

Previously the serializer only quoted strings when plain emission would change
the scalar type, e.g. string `"123"` becoming an integer. That missed strings
that still scan as strings but are invalid YAML syntax when emitted plainly.

Add regression coverage for the reported `%` / `%test` mapping case and related
plain-scalar indicator cases.

* Tighten plain scalar serialization coverage

Remove unreachable tab checks from the plain-scalar validation helper, since
tabs are escaped before this helper runs. Add regression cases for the reachable
`-`, `?`, and `:` plain-scalar edge cases so both valid and invalid forms are
covered.

This keeps the serializer behavior aligned with YAML plain scalar rules while
avoiding dead validation branches.

* Rerun CI

* Quote plain strings with whitespace or flow indicators

Quote string scalars containing any whitespace or any of `,[]{}` to avoid
context-dependent plain scalar output. The serializer does not know whether a
scalar is used as an implicit key or inside a flow collection, so this keeps the
plain-scalar validation conservative.

Update serializer tests for whitespace and mid-string flow indicator cases.

1401 of 1402 branches covered (99.93%)

Branch coverage included in aggregate %.

13 of 13 new or added lines in 1 file covered. (100.0%)

5141 of 5142 relevant lines covered (99.98%)

403.79 hits per line