json: filter open enum JSON values in no_std

[fallintoplace]

What

• Apply ignore_unknown_enum_values in no_std JSON parsing for optional, repeated, and map open-enum fields.
• Filter unknown enum names while keeping valid unknown numeric values as EnumValue::Unknown.

Why

• Give no_std builds the same lenient behavior as std across these field shapes.

Implementation

• Use the global parse option to filter failed enum values.
• Add no_std coverage and include it in CI.

Benchmark

Not applicable.

[github-actions]

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