Unfilter Content-Type param in API logs

What does this MR do and why?

Related: 20963: Unfilter Content-Type param in API logs ... (#584233)

During a recent incident (Commits API returning 400 errors), engineers couldn't debug content-type parsing issues because Content-Type showed as [FILTERED] in logs.

"params": [
        {
            "key": "Content-Type",
            "value": "[FILTERED]"

The content filter was too broad - it filtered any param containing "content", including Content-Type. This MR changes the content filter from a symbol (substring match) to an exact-match regex (/\Acontent\z/i):

  - `content` param → still filtered (may contain sensitive file data)
  - `Content-Type` param → now visible (just a MIME type like `application/json`)

How to set up and validate locally

For the commit endpoint.

1. Make a request to the commit endpoint

➜ curl -s -X POST "http://localhost:3000/api/v4/projects/1/repository/commits" \
    -H "Content-Type: text/plain" \
    -H "PRIVATE-TOKEN: <your-private-token>" \
    -d '{}'

2. Check the log

➜ tail -1 log/api_json.log | python3 -m json.tool | grep -A 3 '"params"'

3 You should see unfiltered Content-Type

"params": [
        {
            "key": "Content-Type",
            "value": "text/plain"

For the file endpoint.

1. Make a request to the file endpoint

➜ curl -s -X POST "http://localhost:3000/api/v4/projects/22/repository/files/test.txt" \
    -H "Content-Type: application/json" \
    -H "PRIVATE-TOKEN: <your-private-token>" \
    -d '{}'

2. Check the log

➜ tail -1 log/api_json.log | python3 -m json.tool | grep -A 3 '"params"'

3 You should see unfiltered Content-Type

"params": [
        {
            "key": "Content-Type",
            "value": "application/json"

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