fix: preserve line breaks in thinking output

[Huarong]

Summary

This change preserves line breaks in text-mode thinking output instead of collapsing all whitespace into a single line.

Problem

agent_thought_chunk content is currently rendered through collapseWhitespace(...), which flattens \n into spaces before printing. This makes multi-line reasoning hard to read and loses structure that agents intentionally send.

Changes

• preserve \n when flushing buffered thought text
• normalize CRLF to LF before rendering
• render multi-line thought output as a [thinking] block instead of a single flattened line
• add a regression test covering blank lines and line breaks in thought chunks

Why this should be separate

This PR only fixes formatting/readability. It does not change when thought chunks are emitted.

Testing

• ./node_modules/.bin/tsc -p tsconfig.test.json
• node --test dist-test/test/output.test.js