fix: propagate num_labels/id2label to text_config in Qwen3_5Config

[s-zx]

What does this PR do?

Fixes num_labels not being propagated from Qwen3_5Config to its text_config when loading via AutoConfig.from_pretrained(model, num_labels=N).

Root cause: Qwen3_5Config.__post_init__ initializes text_config with default label settings, then calls super().__post_init__(**kwargs) which correctly sets self.id2label on the outer config. However, text_config already has the default id2label (2 labels) and never gets updated.

Fix: After super().__post_init__(**kwargs) runs, propagate id2label and label2id from the outer config to text_config when num_labels or id2label was explicitly provided.

Before:

config = AutoConfig.from_pretrained(model_name, num_labels=1)
print(config.num_labels)             # 1
print(config.text_config.num_labels) # 2  ← wrong

After:

config = AutoConfig.from_pretrained(model_name, num_labels=1)
print(config.num_labels)             # 1
print(config.text_config.num_labels) # 1  ← correct

Both modular_qwen3_5.py and the generated configuration_qwen3_5.py are updated.

Fixes #44625

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: qwen3_5

[zucchini-nlp]

Nope, not what we want as per comment under the issue. Also we'd need a bigger PR to sync all multimodal/nested models