Fix symf rewrite failures (#4497)

For some inputs, our LLM query rewrite would produce multiple lines of XML, whereas we expect a single line. This caused us to return no rewritten term, which results in no context.

This change updates the prompt to encourage the LLM to produce a single line. Additional fixes:
* Make sure to split `value` (the main keyword) on whitespace, to avoid duplicated rewritten terms
* If the rewrite fails for whatever reason, return the original string instead of failing the whole search

Relates to CODY-1615