Support unary interceptors in callback-style clients (rpcCall)

[ganesh-tn]

#1519 Unary interceptor is not invoked after upgrading grpc-web from 1.5.0 to 2.0.2

[Vuhag123]

reverify

[Copilot]

Pull request overview

Fixes issue #1519 where unary interceptors registered via unaryInterceptors were silently ignored on callback-style clients (those generated with import_style=commonjs+dts,mode=grpcweb, which call rpcCall). Before this PR, rpcCall only ran the streamInterceptors_ pipeline. The PR adds a new branch in rpcCall that, when the method is unary and at least one unary interceptor is configured, drives the request through the same unaryInterceptors_ chain used by thenableCall/unaryCall and bridges the Promise-based result back to the existing (error, response) callback contract via a proxy stream that queues listeners/cancellation until the interceptor chain produces the real underlying stream.

Changes:

• Add a unary-interceptor branch to rpcCall that builds a Promise-returning initialInvoker (mirroring thenableCall) and runs it through runInterceptors_(..., unaryInterceptors_).
• Introduce an inline proxyStream object so rpcCall can synchronously return a ClientUnaryCallImpl that supports on/removeListener/cancel even though the real stream is only created asynchronously by the interceptor chain.
• Translate the resolved UnaryResponse (or rejection) back into the callback signature (error, responseMessage).

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