fix: Imposter.delete() of an absent imposter succeeds over HTTP but throws embedded (rift#1371)

[EtaCassiopeia]

Deleting a port that has no imposter now behaves the same on both transports: it is a no-op, as in Mountebank and rift#1371.

Transport	Before	After
Remote, rift 0.22.0 (200 {})	succeeded	succeeds
Remote, older engines (404)	threw ImposterNotFound	succeeds
Embedded (rift_delete_imposter → -1)	threw EngineError	succeeds

A delete that fails and leaves the imposter in place still throws EngineError. So does a remote response that is neither 2xx nor 404.

Changes

• RemoteTransport.deleteImposter treats a 2xx or a 404 as success. Every other status goes through mapError. The class javadoc documents the 404 exception.
• FfiCalls.deleteImposter: on -1 it captures the engine error first, because the probe clears last_error. It then calls rift_get_imposter. If the imposter is gone, the delete is a no-op; if it is still registered, the captured error is thrown. A failed engine delete always leaves the imposter registered (manager.rs removes it from the map last). The JDK 21 variant shares these sources.
• Javadoc on Imposter.delete() (which had none) and RiftTransport.deleteImposter states the contract. docs/design/sdk-api.md is updated to match.

Release note (behaviour change)

Imposter.delete() on an imposter that is already gone now returns normally. Before, it threw ImposterNotFound (remote) or EngineError (embedded). To check whether an imposter exists, use rift.imposter(port).

Tests

• RemoteTransportGateTest:
  • deletingAnImposterAnOlderEngine404sIsANoOp replaces the old deleteMissingImposterThrowsImposterNotFound. It failed before this change with ImposterNotFound.
  • New: deletingAnAbsentImposterOnA022EngineIsANoOp.
  • New: aFailedDeleteStillThrows, where a 500 throws EngineError(500).
• EmbeddedTransportIT.deletingAnImposterTwiceIsIdempotent: deletes the same imposter twice, then a port that was never created. It failed before with EngineError: rift_delete_imposter: Imposter not found on port 49152.
• New ImposterDeleteIT (conformance, SPAWN and EMBEDDED): create, delete twice, then rift.imposter(port) is empty.

Verification

RIFT_IT=1 ./mvnw -B -fae verify passes on JDK 21 against the rift 0.22.0 corpus, dylib and binary. ImposterDeleteIT also passes with CONFORMANCE_TRANSPORT=EMBEDDED.

Closes #308

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