fix(huggingface): preserve batching when pad_token_id is zero

[loyce-cheng]

Description

Preserve the configured HuggingFace pipeline batch size when the tokenizer's pad_token_id is 0. The truthiness check treated this valid ID as missing, either reducing the batch size to 1 or overwriting the existing padding token with the EOS fallback.

Changes Made

• Check explicitly for None before applying the missing-padding fallback.
• Add five model-download-free regression cases covering zero/nonzero padding IDs, preservation with and without EOS, and the existing single-batch fallback when both tokens are missing.
• Correct the tiny-BERT integration test to expect its requested batch size of 10, since its padding ID is 0.

Related Issues

Fixes #2251.

Validation

• Before the fix, the two new zero-padding cases fail: one changes batch size 8 to 1, the other overwrites padding ID 0. The other three cases pass.
• After the fix: pytest runtimes/huggingface/tests/test_common.py -k 'not test_load_pipeline' -q selects 29 passing tests, including the existing tiny-BERT cases. The two existing distilgpt2 loader/export cases were not run.
• A separate CPU smoke test using a locally created tiny BERT confirms that two differently sized inputs are processed together in one padded batch after the fix; before it, they run in two separate single-item batches.
• Black, Flake8, git diff --check, and mypy (--follow-imports=silent) pass for the two changed files.

Local testing used Python 3.12 on Windows with the runtime's pinned Transformers 4.41.2. MLServer imports POSIX SIGQUIT on startup, so a local launcher temporarily aliases that constant solely to import the real modules; no server startup or signal handling is tested. This launcher is outside the PR. Linux CI and the full runtime suite remain unverified locally.

Checklist

• Code follows the project's style guidelines
• Targeted tests related to the change pass
• No documentation changes are necessary
• Code is reviewed by at least one other team member
• No breaking changes

[CLAassistant]

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