tests: we should convert ignored doctests to real doctests where possible

many of the existing docstrings are
> \`\`\` rust,ignore

or
> \`\`\`rust,no_run

we should make these compilable in all cases, and runnable where reasonable

Search the repository's Rust docstrings for examples marked `rust,ignore` or `rust,no_run`. Review each example to determine whether it can compile and run safely, then convert applicable cases and verify that the doctest suite passes. Done means eligible examples are no longer unnecessarily ignored while examples requiring `no_run` remain appropriately handled.

Written by the indexing model from the issue text.