[tobias-tengler]

Every subgraph request sent by HttpSourceSchemaClient.ExecuteAsync allocates two async iterators: an outer one that builds and sends the request and stores the whole client request and its cancellation token in its state, and a nested one that reads the response.

This PR builds the GraphQLHttpRequest synchronously in ExecuteAsync, the pattern the native batching path already uses, and hands it to a single iterator that sends it, tracks the transport and reads the results. A failure while building the request now throws from ExecuteAsync itself instead of from the first MoveNextAsync; every execution node already calls it inside its try, and the request batching fallback now catches it so it is still recorded as a per-request batch error.

Measured with a gateway benchmark over real HttpSourceSchemaClients whose HttpMessageHandler returns canned responses: ListWithLookup ({ products { id name price } }, a root fetch plus one batched lookup, 2 sends) and SingleFetch ({ products { id name } }, 1 send). BenchmarkDotNet's allocated column was unreliable for this async benchmark (missing cells, about 1 KB between two runs of the same build), so allocations come from a probe that reads GC.GetTotalAllocatedBytes(precise: true) over 20,000 sequential requests per round, 5 rounds per run, two runs per build. Each allocation cell lists the per-round value of the two runs. Both builds include #10632, so the baseline row is main plus that change.

Method	Version	B/request	Mean (median, σ)
ListWithLookup (2 sends)	main + #10632	44,096 / 44,064	48.3 µs (1.1)
ListWithLookup (2 sends)	PR + #10632	43,280 / 43,312	47.4 µs (1.6)
SingleFetch (1 send)	main + #10632	32,016 / 31,984	19.0 µs (0.5)
SingleFetch (1 send)	PR + #10632	31,592 / 31,624	19.4 µs (0.6)

The PR allocates 392 B less per subgraph send (784 B with 2 sends, 392 B with 1). Time is unchanged within noise.

[github-actions]

Patch coverage

74.2% of changed lines covered (23/31)

File	Covered	Changed	Patch %
…/Fusion.Execution/Execution/Clients/HttpSourceSchemaClient.cs	23	31	74.2% 🔴

Uncovered changed lines (JSON)

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  "sha": "d4b5c0ed2dc7143b62f3a42ce78ab9a72732985b",
  "files": [
    { "path": "src/HotChocolate/Fusion/src/Fusion.Execution/Execution/Clients/HttpSourceSchemaClient.cs", "ranges": [[127, 130], [283, 286]] }
  ]
}

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Project coverage: 59.0% (311549/527844 lines)