fix: preserve stream-read errors and terminal diagnostics

[voodoohop]

• Preserve the original upstream stream-read message in error metrics instead of collapsing it into missing usage.
• Keep the stream ending within the existing 16,000-character error-log budget, even with oversized chunks. Full billing input remains untouched.
• Keep diagnostic helpers outside the main stream and tracking flow. No generation retries, timeout changes, or weaker billing validation.
• Pin the merged gateway fix: fix: preserve upstream stream-read errors gateway#13. Builds on fix: log the tail and a summary of failed text streams #14730.
• Verified: 95 Gen tests, Gen type-check, pinned-version formatting, and a real gateway → Gen → usage-validator connection-reset reproduction pass.