refactor(runtime): make duplex_stream_rx optional in RunOptions

[jaehafe]

What kind of change does this PR introduce?

Refactor — resolves a TODO left by @nyannyacha

What is the current behavior?

RunOptions.duplex_stream_rx is required, so even workers that don't useDeno.serve() (e.g. pure computation, memory tests) must create a dummy mpsc::unbounded_channel just to satisfy RunOptionsBuilder::build().

// Before: unused channel created just to avoid build() error
let (_tx, duplex_stream_rx) = mpsc::unbounded_channel::<DuplexStreamEntry>();
user_rt.run(
    RunOptionsBuilder::new()
        .stream_rx(duplex_stream_rx) // not needed but required
        .build()
        .unwrap(),
)

ref: crates/base/src/runtime/mod.rs:353
// TODO(Nyannyacha): Make this as optional.
let Some(duplex_stream_rx) = duplex_stream_rx else {
    return Err(anyhow!("stream_rx can't be empty"));
};

What is the new behavior?

duplex_stream_rx is now Option in RunOptions. Workers that don't need networking can omit it entirely.

// After: no dummy channel needed
user_rt.run(
    RunOptionsBuilder::new()
        .build()
        .unwrap(),
)

Changes

• RunOptions.duplex_stream_rx: UnboundedReceiver → Option<UnboundedReceiver>
• RunOptionsBuilder::build(): removed forced unwrap, passes Option through
• DenoRuntime::run(): conditionally puts receiver into OpState via if let Some
• op_net_accept: error message improved from "already used" to "not available"
• Removed 5 dummy channel creations in tests

No behavioral change for production callers — managed.rs and user.rs always
provide a receiver. op_net_accept already handles None via try_take.

Additional context

All related tests pass (9/9): cargo test --package base