fix(chat-template): bind pad_token into chat template context

[ermolushka]

Problem

ChatTemplate deserializes pad_token from tokenizer_config.json, but only
bos_token, eos_token, and unk_token are forwarded into the MiniJinja
render context in apply_chat_template. Any chat template that references
pad_token (e.g. {% if pad_token is defined %}...{% endif %}) always sees
it as undefined, diverging from Transformers, which binds it.

Fixes #2432.

• Added a pad_tok() accessor on ChatTemplate, mirroring the existing
  bos_tok() / eos_tok() / unk_tok().
• Added a pad_tok parameter to apply_chat_template_to and bound
  it into the MiniJinja context as pad_token, together with the other special
  tokens, in both the tools and no-tools render branches.
• Extracted pad_tok from chat_template.pad_token in processing.rs
  the same way the other three tokens already are.

Testing

• Added pad_token_is_bound_into_template_context and
  pad_token_is_bound_in_chat_template_context (the latter matching the
  regression test from the issue) in mistralrs-core/src/pipeline/chat_template.rs.
• cargo test -p mistralrs-core --lib pipeline::chat_template (22/22 passing)
• cargo clippy --workspace --tests --examples -- -D warnings clean
• cargo fmt --all -- --check clean

[github-actions]

Code Metrics Report

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 Markdown                283        13044            0         9784         3260
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 |- Python               135         7349         6119          306          924
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 (Total)                            25038         9662        10536         4840
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 Rust                    772       500427       456580         5582        38265
 |- Markdown             443        10396          452         8755         1189
 (Total)                           510823       457032        14337        39454
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 Svelte                   19         1974         1832           50           92
 |- CSS                    1            4            4            0            0
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 (Total)                             2906         2609           75          222
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