Add interpreter-wide R help search support

[kv9898]

This is the R side prerequisite for posit-dev/positron#15923, to resolve posit-dev/positron#422.

Summary

• add Help comm requests for interpreter-wide search and autocomplete topics
• use utils::help.search() to search documentation across installed R packages, including unloaded packages
• return package-qualified topic suggestions so duplicate aliases open the intended documentation
• add integration coverage for search requests and topic suggestions

Verification

• cargo +nightly fmt --all --check
• cargo check --package ark
• manually verified R search results and package-qualified topic selection in Positron

The targeted integration test reaches the new search handler locally, but cannot complete with the current system R installation because /usr/lib/R/doc/KEYWORDS.db is missing.