Handle client-cancelled shape requests as disconnects instead of 500s

[alco]

Fixes #4790

Two complementary changes:

1. Abort the long poll the moment the cancel arrives. wait_for_live_change/2 now also matches Bandit's {:bandit, {:rst_stream, _}} notification and completes the request immediately — status 499, empty body, change-listener cleanup — releasing the process and admission permit at cancel time. The message format is Bandit-private (Bandit.HTTP2.Stream.deliver_rst_stream/2); the receive clause carries a comment pinning that. HTTP/1 has no equivalent early signal and is covered by the second change only.

2. Classify send-time transport failures as client disconnects. A %Bandit.TransportError{error: :closed} caught around the plug pipeline is now accounted for as status 499 (nginx's "client closed request" convention) with a shape_req.client_disconnected span attribute — no record_exception, no unreadable error body — and is reraised for Bandit to close the stream through its native quiet path (log_client_closures: false by default). This mirrors how the chunked streaming path already treats {:error, "closed"} as a quiet halt.

The [:electric, :plug, :serve_shape] telemetry event still fires for these requests (with status 499), so cancellation volume stays measurable while dropping out of the 5xx error signal.

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https://claude.ai/code/session_01C4Hiytggpfq8UqvFnshh5r

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
2332	1	2331	2371

View the top 2 failed test(s) by shortest run time

Elixir.Electric.Shapes.FilterTest::test optimisations where clause in the form `array_field @> const_array` is optimised

Stack Traces | 0.0478s run time

11) test optimisations where clause in the form `array_field @> const_array` is optimised (Electric.Shapes.FilterTest)
     .../electric/shapes/filter_test.exs:894
     Assertion with < failed
     code:  assert add_reductions < max_reductions
     left:  7980
     right: 6500
     stacktrace:
       (elixir 1.20.2) lib/enum.ex:983: Enum."-each/2-lists^foreach/1-0-"/2
       .../electric/shapes/filter_test.exs:922: (test)

test/runtime-dsl.test.ts > F: coordination orchestration > F1: dispatcher routes to the requested specialist type and records the child

Stack Traces | 0.137s run time

AssertionError: expected false to be true // Object.is equality

- Expected
+ Received

- true
+ false

 ❯ test/runtime-dsl.test.ts:3936:7

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[github-actions]

This PR has been released! 🚀

The following packages include changes from this PR:

• @core/sync-service@1.8.0

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