fix(core): handle malformed Unicode when writing agent state files (#2255)

Fixes #2204.

`JsonFileAgentStateStore` could throw an `UnmappableCharacterException`
when serialized state contained malformed UTF-16 input, such as an
unpaired surrogate (`\uD800`). This caused state persistence to fail and
could interrupt `HarnessAgent.streamEvents()`.

This PR:
- Adds a UTF-8 writer configured to replace malformed or unmappable
input with the Unicode replacement character (`\uFFFD`).
- Applies the writer to single-state atomic writes, full list rewrites,
and list appends.
- Keeps the existing `JsonCodec.toPrettyJson()` and `JsonCodec.toJson()`
serialization behavior unchanged.
- Adds tests covering malformed Unicode in single-state and list-state
persistence.