Normalize 4xx responses for invalid plugins/actions/assets and quiet their logging

[sgiehl]

Description

• Map client-side faults to consistent 4xx responses: missing/invalid API params return 400, invalid actions return
  404, missing chunks return 404, missing plugins return 404, deactivated plugins return 403.
• Silence noisy error logging for 4xx-only failures by tagging these paths as HttpCodeException and downgrading log
• System/integration tests added for HTTP status expectations (400 missing param, 404 missing plugin, 404 invalid
  action, 404 missing asset chunk) and for plugin activation checks (404 missing plugin, 403 deactivated) plus missing
  chunk error in the asset fetcher.

Checklist

• [✔] I have understood, reviewed, and tested all AI outputs before use
• [✔] All AI instructions respect security, IP, and privacy rules

Review

• Functional review done
• Potential edge cases thought about (behaviour of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• New documentation added or existing documentation updated

[github-actions]

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