fix: send HTML tag handling to DeepL so notranslate works

[Halvanhelv]

The DeepL provider sent no tag_handling. Per DeepL's documentation, "Without tag_handling, tags are treated as regular text" — and class="notranslate", which DeepL does honour, is honoured only under HTML tag handling.

The tokenizer hands a notranslate span to the provider whole, tags included, precisely so the provider can leave its contents alone. It could not.

Confirmed against the live API

"<span class='notranslate'>Bold Mountain</span> is a good place."

no tag_handling ->  "<span class='notranslate'>Болд-Маунтин</span> — отличное место."
tag_handling    ->  "<span class='notranslate'>Bold Mountain</span> — это хорошее место."

Note what the first line does: the span's tags survive untouched and only the protected name is translated. Nothing about the output looks wrong — which is why this went unnoticed. tag_handling appears nowhere in this repository's history, so notranslate has not worked for a single day since the gem moved from Google to DeepL.

The change

tag_handling: :html and tag_handling_version: "v2" (the algorithm DeepL's docs recommend) are sent as defaults a caller can override, the way every other provider option works.

Under HTML tag handling DeepL defaults split_sentences to nonewlines; this library sends one sentence per value, so that changes nothing here.

Detection is left alone: it sends a 100-character sample that may end mid-tag, and DeepL never strictly parses HTML anyway.

Verification

• 229 runs, 492 assertions, 0 failures, on seeds 1, 99 and 4242. RuboCop clean on 52 files.
• Live end-to-end through the gem:

"<span class='notranslate'>Bold Mountain</span> is a good place."
→ "<span class='notranslate'>Bold Mountain</span> — это хорошее место."

"<!-- internal note --> Visible text here."
→ "<!-- internal note --> Здесь отображается текст."

"Salt &amp; pepper. Fine."
→ "Соль и перец. Хорошо."