Make the e2e write phase work live across all five clients

Running the matrix against a live gateway surfaced several issues:

- The write phase is now opt-in (-VerifyWrite, was -SkipWrite). It runs
  right after register so only a small event backlog precedes the write,
  and asserts the reliable OnWriteComplete signal (the written value is
  not echoed back by a provider-driven attribute like TestChangingInt, so
  the value compare is best-effort).
- Java was launched as bare "gradle", which .NET's Process.Start cannot
  exec (it is gradle.bat) — resolve the launcher and run it via cmd.exe.
- The Java client's MxEventStream queue capacity was 16, which overflows
  on any active session's backlog-replay burst; raised to 1024.
- The Rust stream-events CLI now renders the event family as the proto
  enum name, matching the protobuf-JSON the other four clients emit.

Update docs/GatewayTesting.md for the reworked write phase.

Verified live: the full five-client matrix passes with -VerifyWrite.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/GatewayTesting.md

@@ -180,19 +180,30 @@ path and writes a JSON report under `artifacts/e2e/`:
 2. **Bulk** — verifies `SubscribeBulk` / `UnsubscribeBulk` on a bounded tag
    subset (skip with `-SkipBulk`).
 3. **Add-item / advise** — adds and advises every discovered test tag.
-4. **Write round-trip** — writes a per-client sentinel value to a configurable
-   writable attribute (`-WriteAttribute`, default `TestChangingInt`), then
-   asserts the same value is echoed back through the event stream. Skip with
-   `-SkipWrite`. The Rust `stream-events` CLI emits full per-event JSON
-   (`itemHandle` + `value`) so all five clients run an identical value compare.
-5. **Stream** — asserts a bounded event stream delivers at least one event
+4. **Stream** — asserts a bounded event stream delivers at least one event
    (skip with `-SkipStream`).
-6. **Parity** — asserts MXAccess error paths are rejected rather than silently
+5. **Parity** — asserts MXAccess error paths are rejected rather than silently
    succeeding: an invalid item handle and an unknown session id (skip with
    `-SkipParity`).
-7. **Auth rejection** — asserts `open-session` is rejected when the API key is
+6. **Auth rejection** — asserts `open-session` is rejected when the API key is
    missing, and (when `-RejectScopeApiKeyEnv` names an insufficient-scope key)
    when the key lacks the required scope. Skip with `-SkipAuth`.
+7. **Write round-trip** — *opt-in (`-VerifyWrite`).* Runs right after
+   `register`: adds and advises a configurable writable attribute
+   (`-WriteAttribute`, default `TestChangingInt`), writes a per-client
+   sentinel value, then streams events and asserts an `OnWriteComplete` event
+   for that item is observed — proof the write round-tripped through the
+   gateway, worker, and MXAccess provider. The written value being echoed back
+   in an `OnDataChange` is recorded best-effort (`echoObserved`): a
+   provider-driven attribute such as `TestChangingInt` accepts the write but
+   immediately overwrites it, so no data-change carries the value back. The
+   Rust `stream-events` CLI emits full per-event JSON (`family`, `itemHandle`,
+   `value`) so all five clients apply the same checks.
+
+   It is opt-in because it mutates live tag state. The phase fails fast if the
+   write command is rejected — e.g. against a gateway whose worker predates
+   write support (`MxAccessCommandExecutor` returning `InvalidRequest` for
+   `Write`/`Write2`/`WriteSecured`/`WriteSecured2`).
 
 Build the gateway and worker, start the gateway, and provide a valid API key
 before running the client e2e script:
@@ -209,9 +220,9 @@ powershell -ExecutionPolicy Bypass -File scripts/run-client-e2e-tests.ps1 -Clien
 powershell -ExecutionPolicy Bypass -File scripts/run-client-e2e-tests.ps1 -BulkTagCount 10
 powershell -ExecutionPolicy Bypass -File scripts/run-client-e2e-tests.ps1 -SkipStream
 powershell -ExecutionPolicy Bypass -File scripts/run-client-e2e-tests.ps1 -SkipBulk
-# Write round-trip: point at a writable scalar attribute and its value type.
-powershell -ExecutionPolicy Bypass -File scripts/run-client-e2e-tests.ps1 -WriteAttribute TestChangingInt -WriteType int32
-powershell -ExecutionPolicy Bypass -File scripts/run-client-e2e-tests.ps1 -SkipWrite
+# Write round-trip (opt-in): point at a writable scalar attribute and its
+# value type.
+powershell -ExecutionPolicy Bypass -File scripts/run-client-e2e-tests.ps1 -VerifyWrite -WriteAttribute TestChangingInt -WriteType int32
 # Auth rejection: also assert an insufficient-scope key is denied.
 powershell -ExecutionPolicy Bypass -File scripts/run-client-e2e-tests.ps1 -RejectScopeApiKeyEnv MXGATEWAY_READONLY_API_KEY
 # Run all five clients concurrently as isolated child processes.
@@ -221,11 +232,12 @@ powershell -ExecutionPolicy Bypass -File scripts/run-client-e2e-tests.ps1 -DryRu
 powershell -ExecutionPolicy Bypass -File scripts/run-client-e2e-tests.ps1 -Endpoint localhost:5000 -ApiKeyEnv MXGATEWAY_API_KEY
 ```
 
-The write round-trip fails loudly if `-WriteAttribute` does not name a writable
-scalar attribute, if the write is rejected, or if the sentinel value is not
-observed within `-WriteEchoMaxEvents` (default 200) streamed events. Point
-`-WriteAttribute` at a stable writable attribute, raise `-WriteEchoMaxEvents`,
-or pass `-SkipWrite` if no suitable attribute is deployed.
+When `-VerifyWrite` is enabled, the write round-trip fails loudly if the write
+command is rejected, if `-WriteAttribute` does not name a writable scalar
+attribute, or if no `OnWriteComplete` event is observed for the written item
+within `-WriteEchoMaxEvents` (default 200) streamed events. Raise
+`-WriteEchoMaxEvents` if the gateway's per-session event backlog is large
+enough to push `OnWriteComplete` past that bound.
 
 ## Focused Commands