add unit test coverage for FutureBijection and ElidableStatement

[aglinxinyuan]

Background

Two utility modules in engine/common currently lack a dedicated unit-spec:

Source class	Purpose
FutureBijection	Implicit-class extensions converting between Twitter Future and Scala Future (both success and failure paths) — used wherever the engine bridges twitter-util-based RPC with scala-future-based code
ElidableStatement	Tiny logging helper that wraps by-name blocks in scala.annotation.elidable so they compile away below the configured level

Behavior to pin

FutureBijection

Surface	Contract
TwitterFuture.value.asScala	resolves to the same value
TwitterFuture.exception.asScala	resolves with the same Throwable (preserves exception type + message)
ScalaFuture.successful.asTwitter	resolves to the same value
ScalaFuture.failed.asTwitter	resolves with the same Throwable
both directions	preserve null-vs-value semantics on the value path
Twitter → Scala → Twitter and Scala → Twitter → Scala	round-trip values and exceptions

ElidableStatement

The texera build sets -Xelide-below WARNING (amber/build.sbt). Every ElidableStatement helper is annotated with an elide level strictly below WARNING (FINEST / FINER / FINE / INFO), so the Scala compiler replaces every CALL to these helpers with a () Unit value at compile time. The spec pins this silent-in-production contract:

Surface	Contract
info / fine / finer / finest (block executes side effect)	the side effect MUST NOT fire (block is elided)
same methods (block throws)	the exception MUST NOT propagate (block is elided)
1000 successive elided calls	the side-effect counter remains at 0
return type	Unit (compile-time enforced)
parameter shape	accepts a => Unit by-name block (compile-time enforced)

A regression that bumped a method's elide level above WARNING, removed the @elidable annotation, or relaxed -Xelide-below in the build would re-enable side effects in production — and this spec would catch it.

Scope

• New spec files (one per source class per the spec-filename convention):
  • FutureBijectionSpec.scala
  • ElidableStatementSpec.scala
• No production-code changes.
• FutureBijection spec uses Await.result / TwitterAwait.result with a short timeout to drive the underlying futures to completion.