fix: only treat HTTP 4xx+ responses as errors

[sohankshirsagar]

Note

Aligns error status handling across instrumentations and expands acceptable response types.

• Change error threshold from >=300 to >=400 when setting span status in fetch, http (client/server), and nextjs instrumentations
• Update replay/exported span status fields to use >=400 for StatusCode.ERROR
• Add DecodedType.HTML to ACCEPTABLE_CONTENT_TYPES to avoid blocking HTML responses

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[cubic-dev-ai]

No issues found across 4 files