Fix Qwen3.5 num_labels not propagated to text_config

[gambletan]

Summary

• Fixes num_labels (and id2label/label2id) not being propagated from the outer Qwen3_5Config to its inner text_config when passed via AutoConfig.from_pretrained(..., num_labels=1).
• When text_config is None or a dict, label-related kwargs are now forwarded using setdefault (so explicitly provided text_config values take precedence).
• Both the modular source (modular_qwen3_5.py) and the auto-generated config (configuration_qwen3_5.py) are updated.

Reproduction (from issue)

from transformers import AutoConfig

config = AutoConfig.from_pretrained(
    "onnx-internal-testing/tiny-random-Qwen3_5ForConditionalGeneration",
    num_labels=1,
)
print(config.num_labels)             # 1
print(config.text_config.num_labels) # 2  <-- should be 1

Fix

Before text_config is instantiated, propagate num_labels, id2label, and label2id from kwargs into text_config (as a dict) using setdefault, so that user-supplied values reach the inner config without overriding explicit text_config entries.

Fixes #44625

Test plan

• Verify AutoConfig.from_pretrained(model, num_labels=1) sets config.text_config.num_labels == 1
• Verify explicitly passing text_config={"num_labels": 3} still takes precedence
• Verify default behavior (no num_labels kwarg) is unchanged (text_config.num_labels == 2)

[github-actions]

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

run-slow: qwen3_5