fix: preserve upstream stream-read errors

[voodoohop]

• Emit the original stream-read error as a complete SSE error event instead of silently closing the response.
• Keep error writing in one helper; existing stream loops only call it from their catch blocks. No retries or timeout changes.
• Preserve partial output and healthy streams; covers standard and Bedrock readers.
• Verified: 9 stream tests, 14 parameter tests and build pass. Full type-check output matches clean main exactly.
• CI exceptions match merged fix: preserve explicit chat provider parameters #12: broken documentation links and formatting in the same six untouched files. Changed-file formatting passes.
• Companion Pollinations update records this error and retains bounded terminal diagnostics.