Fix TypeError on list-typed ignore_keys_at_rope_validation in RoPE config

[Charly21r]

What does this PR do?

Fixes #46121

RotaryEmbeddingConfigMixin.ignore_keys_at_rope_validation is a set at the class level, but JSON has no set type, so any config.json that serializes this field (e.g. checkpoints written by LoRA merge / export tooling like ms-swift) loads it back as a list instance attribute that shadows the class default. RotaryEmbeddingConfigMixin.convert_rope_params_to_dict then does:

self.ignore_keys_at_rope_validation = self.ignore_keys_at_rope_validation | {"partial_rotary_factor"}

which raises TypeError: unsupported operand type(s) for |: 'list' and 'set' whenever partial_rotary_factor is also set on the config. In practice this prevents serving such merged checkpoints (observed downstream in vLLM with merged Qwen3.5 checkpoints).

This PR coerces the attribute to a set before the union in src/transformers/modeling_rope_utils.py, and adds a regression test in tests/utils/test_modeling_rope_utils.py covering both direct attribute assignment and the from_dict round-trip path that mirrors the JSON-deserialization flow.

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Who can review?

@Rocketknight1

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https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=46142&sha=1267e6

[Rocketknight1]

LGTM, CI issues seem unrelated. It's core code though, so I'll wait for a core maintainer to approve in case I'm totally wrong here! cc @ArthurZucker @Cyrilvallez @vasqu