feat: signals API and id-backed InvocationHandle

[slinkydeveloper]

Summary

Adds signals to the Rust SDK — named, one-shot durable promises scoped to an invocation, matching the feature already shipped in the TypeScript, Python, and Go SDKs. Built on the create_signal_handle / sys_complete_signal / sys_attach_invocation syscalls that were already present (unused) in restate-sdk-shared-core 0.10.0, so no dependency bump.

A handler awaits a signal by name; any other handler completes it by targeting the awaiter's invocation id + signal name — i.e. "awakeables addressed by (invocation_id, name)", with no opaque token to exfiltrate.

// await side (any context; a DurableFuture, so it works in select!)
let approved: bool = ctx.signal::<bool>("approved").await?;

// send side (fluent, sync fire-and-forget)
ctx.invocation_handle(id).signal("approved").resolve(true);
ctx.invocation_handle(id).signal("approved").reject(TerminalError::new("nope"));

InvocationHandle redesign (breaking)

To make the signal send-side ergonomic (and generally cleaner), InvocationHandle becomes an id-backed struct with synchronous accessors:

• invocation_id() -> &str (was async -> Result<String, _>)
• cancel() — sync fire-and-forget (was async)
• attach::<T>() — new, awaits the invocation's output (sys_attach_invocation)
• signal(name) — new, returns a SignalHandle with resolve/reject

Consequently:

• send() / send_after() now return an awaitable SendHandle (impl IntoFuture → InvocationHandle). The send is issued eagerly, so bare client.foo().send(); still fire-and-forgets. IntoFuture (rather than a bare impl Future) keeps fire-and-forget free of the must-use lint and keeps the eager send/lazy-id split clean.
• call() still awaits to the response and gains invocation_handle(&self) (takes &self, does not consume the future); it retains a #[deprecated] invocation_id() to soften the break. CallFuture drops its InvocationHandle supertrait.

Test-services / e2e

• createSignal command added to VirtualObjectCommandInterpreter.
• resolveSignal / rejectSignal handlers added to TestUtilsService.

Matches the contract implemented in the other SDKs so the shared e2e suite exercises the flows.

Example

New examples/signals.rs (await / resolve / reject).

Verification

• cargo build --workspace --all-features --all-targets ✅
• cargo clippy --workspace --all-features --all-targets -- -D warnings ✅
• unit + integration + doc tests ✅ (the new # Signals doc examples compile)
• Docker-based e2e (testcontainers) not run locally.

🤖 Generated with Claude Code

[github-actions]

Test Results

  8 files  ±0    8 suites  ±0   2m 30s ⏱️ -14s
 43 tests ±0   43 ✅ ±0  0 💤 ±0  0 ❌ ±0 
186 runs  ±0  186 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit a35f67d. ± Comparison against base commit 4dd2de4.