[Test] Fix 6 skipped + 1 xfailed architecture export tests

Parent Issue

Subtask of #203

Problem

7 architectures in tests/export/test_all_architectures_io.py cannot be fully validated:

• 6 skipped: AutoConfig.for_model() returns no config class (composite/meta architectures)
• 1 xfailed: Optimum bug in speecht5 OnnxConfig

Skipped Architectures (6)

These architectures require manual HF config construction because AutoConfig.for_model() doesn't support them directly.

1. internlm2 (text_decoder)

• Reason: Not registered in transformers AutoConfig mapping
• Fix: Create InternLM2Config manually or use LlamaConfig as base (InternLM2 shares Llama architecture)

2. siglip-text-with-projection (multimodal)

• Reason: Sub-component of SigLIP, not a standalone model
• Fix: Extract text config from SiglipConfig via siglip_config.text_config

3. siglip-text (multimodal)

• Reason: Sub-component of SigLIP
• Fix: Same as above - extract from parent config

4. musicgen (audio)

• Reason: Composite model with text encoder + audio decoder
• Fix: Create MusicgenConfig with sub-configs: MusicgenDecoderConfig + T5Config

5. vision-encoder-decoder (seq2seq)

• Reason: Meta architecture requiring encoder + decoder configs
• Fix: Create VisionEncoderDecoderConfig from ViTConfig (encoder) + GPT2Config (decoder)

6. encoder-decoder (seq2seq)

• Reason: Meta architecture requiring encoder + decoder configs
• Fix: Create EncoderDecoderConfig from BertConfig (encoder) + BertConfig (decoder)

XFailed Architecture (1)

7. speecht5 (audio)

• Reason: SpeechT5OnnxConfig._behavior is not set when task="text-to-audio" with default SpeechT5Config
• Root cause: Optimum's SpeechT5OnnxConfig.__init__ expects a specific _behavior attribute that the default config doesn't provide
• Fix options:
  • A) File upstream Optimum bug report
  • B) Construct config with correct _behavior attribute manually
  • C) Override with ModelKit's own register_onnx_overwrite

Acceptance Criteria

• All 6 skipped tests converted to PASSED
• speecht5 xfail resolved (either fixed or documented as upstream bug)
• Zero regressions in existing 144 passing tests
• Total: 151/151 passing (0 skipped, 0 xfailed)

Files to Modify

• tests/export/test_all_architectures_io.py - Add manual config construction for skipped architectures
• tests/export/conftest.py - Add new config fixtures if needed