[flutter_tools] Stream web dev server proxy responses instead of buffering them - #194934

When a request matches a web_dev_config.yaml proxy rule, _applyProxyRules returns the Response that shelf_proxy builds around the backend stream. That response has no context, and shelf_io only touches HttpResponse.bufferOutput when shelf.io.buffer_output is in the context, so proxied responses go through dart:io's default 8 KB output buffer. A single SSE event or NDJSON line just sits in that buffer until it fills up or the backend closes the response. In the issue's repro the first line shows up after 33 ms when hitting the backend directly, and doesn't arrive through the proxy within the 3 second timeout.

The fix sets shelf.io.buffer_output: false on the proxied response via Response.change. Status, headers and the streamed body carry over unchanged, only the context gets the extra key, so shelf_io writes each chunk out as soon as it comes in from the backend. I didn't scope it to text/event-stream or NDJSON: dart:io already writes chunks over 8 KB straight through even with buffering on, so for regular responses the only difference is that small chunks no longer get coalesced. Both handlers in web_asset_server.dart (release/Wasm and the DWDS debug one) go through proxyMiddleware, so both get this.

The new test in proxy_test.dart has the backend send one line and only finish the response after that line has made it through the proxy. Without the fix the first line never reaches the client and the test times out. With it, all 40 tests in proxy_test.dart pass, and so does the rest of test/general.shard/web/. I also ran a local check (not part of this PR) with an SSE backend behind WebAssetServer.start: the first event came through after 24 ms in release and 141 ms in Wasm debug. Without the change it took about 8 s, which is when that backend closed the response.

dart-lang/shelf#503 asks for a bufferOutput option on proxyHandler for the same SSE case. Even with that option the tool would still have to opt in here, and shelf.io.buffer_output is the key shelf_io already documents for this, so there's no need to wait for a shelf_proxy release. If that option lands later, this call site can switch over to it.

Fixes #194354

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