fix: surface parse errors in chat content streaming

[steve-calvert-glean]

Summary

• contentOnly() previously silently skipped all JSON parse errors, producing empty output with exit code 0 on a fully corrupt stream
• Now returns an error when no content could be parsed, while still tolerating mixed valid/corrupt lines
• Updated existing test that expected silent success to correctly expect an error

Test plan

• Unit test: valid NDJSON returns content, no error
• Unit test: all-corrupt NDJSON returns error
• Unit test: mixed valid/corrupt returns content from valid lines
• Unit test: empty input returns empty string, no error
• mise run test:all passes (455 tests, lint clean, binary builds)