fix(workflowengine): stricter length header handling

[joshtrichards]

• Resolves: [Bug]: Malformed request to FileSize check causes TypeError, leading to Denial of Service #55276

Summary

Should fix #55276 and also tidies things up a bit more for future maintenance.

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

[joshtrichards]

/backport to stable32

[joshtrichards]

/backport to stable31

[joshtrichards]

/backport to stable30

[backportbot]

The backport to stable30 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable30
git pull origin stable30

# Create the new backport branch
git checkout -b backport/55283/stable30

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 51318979 f5039750

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/55283/stable30

Error: Failed to push branch backport/55283/stable30: remote: {"auth_status":"auth_error","body":"Invalid username or token. Password authentication is not supported for Git operations."}
fatal: Authentication failed for 'https://github.com/nextcloud/server.git/'

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.