Users can now use booleans and numbers for specific configuration
fields, providing a more concise way to configure the assistant's
behavior. For instance, `inline_results` now accepts `true` (full),
`false` (off), or a number to specify line truncation. Similarly,
`link_style` fields now accept `true` for full links and `false` to
disable them.

This change also renames the `is_default` property to
`discard_when_merged` within `MergeableString` and `MergeableVec`. This
new name more accurately describes the property's role: ensuring that
initial "default" configurations are completely replaced when a user
provides their own settings, regardless of the chosen merge strategy.

Finally, `serde-untagged` has been added to the workspace dependencies,
to allow for better error messages when deserializing untagged enums.
This has not been applied to all enums, but will be done in a future
commit.

Signed-off-by: Jean Mertz <git@jeanmertz.com>