[Bug] Proxy gRPC converter should tolerate malformed timer properties

### Bug description

`GrpcConverter.buildSystemProperties` parses timer message properties with `Long.parseLong` when building the gRPC `SystemProperties.delivery_timestamp` field:

- `MessageConst.PROPERTY_TIMER_DELAY_SEC`
- `MessageConst.PROPERTY_TIMER_DELIVER_MS`

If either property is present but malformed, `buildMessage` can throw `NumberFormatException` while converting a Broker `MessageExt` into the gRPC v2 `Message` response. This can make a single malformed timer property fail the whole gRPC message response/query conversion path.

### Expected behavior

The converter should tolerate malformed optional timer properties. Valid timer properties should still set `delivery_timestamp`; malformed values should be ignored with a diagnostic warning instead of failing the message conversion.

### Affected area

`proxy` module, gRPC v2 message conversion.

### Suggested fix

- Parse timer properties through a safe helper.
- Keep existing behavior for valid `PROPERTY_TIMER_DELAY_SEC` and `PROPERTY_TIMER_DELIVER_MS` values.
- Ignore malformed values and log a warning with diagnostic context.
- Add unit tests for valid and malformed timer properties.

Start in the proxy module at GrpcConverter.buildSystemProperties and follow its use from buildMessage. Check how PROPERTY_TIMER_DELAY_SEC and PROPERTY_TIMER_DELIVER_MS are parsed, then add unit tests covering valid and malformed values. Done means valid properties still set delivery_timestamp while malformed optional values are ignored with a warning and conversion continues.

Written by the indexing model from the issue text.