fix: send non_splitting_tags and ignore_tags as arrays in JSON payloads

[modrictin]

🛠️ What this fixes:

We got a Http error occurred (400) error when migrating from 2.x to 4.0.

This PR corrects how non_splitting_tags, ignore_tags are handled when building the DeepL translation request body.

Previously, these were passed as comma-separated strings via:

implode(',', $this->translation->getNonSplittingTags())
implode(',', $this->translation->getIgnoreTags())

This worked when the request body was encoded as application/x-www-form-urlencoded, but the current implementation builds a JSON request body using a stream and sets the Content-Type to application/json.

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✅ What this PR does:

• Passes non_splitting_tags and ignore_tags as proper arrays in the JSON body (instead of comma-separated strings)
• Ensures compatibility with DeepL’s stricter input validation on application/json payloads
• Fixes unexpected Http error occurred (400) when setting non_splitting_tags or ignore_tags

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🧪 Verified behavior:

We confirmed this fix by sending the following test input:

<par>The firm said it had been </par><par> conducting an internal investigation.</par>

With:

'non_splitting_tags' => ['par']

✅ Result (correct behavior):

<par>Das Unternehmen teilte mit, dass es </par><par> eine interne Untersuchung durchgeführt habe.</par>

Without non_splitting_tags, DeepL treated the tags as sentence boundaries, leading to split context and degraded translation quality.

The same principle applies to ignore_tags, which must also be passed as an array when using JSON payloads.

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⚠️ Important Note / Incomplete Fix:

This PR only addresses the issue in the DeeplTranslationRequestHandler.

However, the same incorrect handling of list-based parameters (via implode(...)) appears to be present in at least:

• DeepBatchTranslationRequestHandler

There may be other request handlers or tests affected as well — I haven’t reviewed all of them.

🚧 This PR is not a full fix across the entire package. Contributions are welcome to complete this.

P.S. 'tag_handling' can be affected as well, but I don't think so.

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✅ Summary:

This update brings the client in line with DeepL API expectations for JSON requests. It resolves a backend-breaking change and ensures future-proof compatibility with non_splitting_tags, ignore_tags, and other list-style fields.

[usox]

Hi @modrictin

Thank you for your contribution and the detailed description. This is looking good so far, we will have a look at the remaining payloads and will cover that within #93