fix(view source): put a space after the colon on a rest parameter - #19024

Description

view source renders a rest parameter without a space after the colon, unlike
every other parameter kind:

> def test [a?:any --b:int ...rest:string] { echo 'test' }; view source test
def test [ a?: any --b: int ...rest:string] { echo 'test' }

a?: any and --b: int are spaced; ...rest:string is not. In render_def
the branches are:

The rest branch is the only one missing the space. The command's own examples
already show ...rest: string, in two of them, so the documented form was the
right one and the code disagreed with it.

Why it was not caught: view_source.rs declares eight example results and had
no test running them. Adding test_examples for ViewSource is what surfaced
this, and it also caught a second, smaller mismatch — the module example
claimed

" export-env { $env.FOO_ENV = 'BAZ' }"

while view source returns the source between the module's braces verbatim,
so the real value has the trailing space too. That one is the example being
wrong rather than the code, so the example is corrected.

User-facing changes (Release notes)

view source now renders a rest parameter as ...rest: string rather than
...rest:string, matching how it already renders every other parameter.

Tests + Formatting

Added test_examples for ViewSource, which runs all eight declared example
results. It is not vacuous — it is what found both mismatches, failing first on

expected: "def --env --wrapped foo [ ...rest: string] { print $rest }"
got:      "def --env --wrapped foo [ ...rest:string] { print $rest }"

and then on the module example. Both are green now.

The full nu-command lib suite (815 tests) and the existing view_source
integration tests pass. I checked nothing asserted the old spelling: the only
other ...name:type in the tree is input source in an unrelated parser test,
not an assertion on view source output. cargo fmt --all -- --check and
cargo clippy --all-targets are clean.

Additional notes

Third small nu-command fix from me after #19017 and #19023, same theme: a
declared example result that nothing was checking.