test(e2e): cover Deno module generation

[TomChv]

Deno was wired but never exercised: it is the one branch in generation that
depends on the runtime, since Deno resolves imports through deno.json rather
than node_modules plus tsconfig paths. Generating a Deno module as if it were a
Node one would produce a tree that looks complete but leaves the runtime unable
to resolve @dagger.io/dagger at all.

The check asserts both directions — the deno.json entries are written and the
user's own are preserved, and no package.json or tsconfig.json is produced.
Confirmed beyond the assertions by generating the fixture and calling it:
dagger call hello returns through the engine's Deno runtime.

No separate staleness check: the engine already surfaces each @generate
function as a check that fails when it would produce changes, which is exactly
that, with a better message. Verified by tampering with the committed bindings
and watching packager:library-bindings fail. Recorded in the design instead.

Signed-off-by: Tom Chauveau tom@dagger.io

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