Make tunnel-client max HTTP content length configurable (#3034)

[daguimu]

Problem

When the Arthas Web Console is connected through a tunnel-server and a user clicks Download on a JFR recording larger than 10 MB, the download silently fails and the browser receives a body containing the literal text error. The tunnel-server logs:

com.alibaba.arthas.deps.io.netty.handler.codec.http.TooLongHttpContentException:
    Response entity too large: DefaultHttpResponse(decodeResult: success, version: HTTP/1.1)
  at io.netty.handler.codec.http.HttpObjectAggregator.handleOversizedMessage(HttpObjectAggregator.java:275)

Recordings up to 10 MB download fine; anything larger is truncated to the error string.

Root Cause

tunnel-client/ProxyClient configures Netty's HttpObjectAggregator with the shared ArthasConstants.MAX_HTTP_CONTENT_LENGTH = 10 MB constant. When the agent's local Arthas HTTP server emits a response exceeding 10 MB (such as a JFR file), the aggregator rejects it before the bytes can reach the tunnel-server, and the proxy falls back to its error payload.

Fix

Introduce a JVM system property arthas.tunnel.client.max-http-content-length (in bytes) that overrides the cap used by ProxyClient only. The shared MAX_HTTP_CONTENT_LENGTH constant is left untouched, so unrelated consumers (WebSocket frame limit, ObjectView size limit, etc.) keep their existing semantics.

• Default remains 10 MB - existing deployments behave identically.
• Setting -Darthas.tunnel.client.max-http-content-length=104857600 (100 MB) lets users download larger JFR recordings.
• Invalid values (non-numeric, zero, negative) fall back to the default rather than failing fast, matching the lenient parsing already used elsewhere in arthas-common.

Files touched:

• common/src/main/java/com/taobao/arthas/common/ArthasConstants.java - add property name + getTunnelClientMaxHttpContentLength() helper.
• tunnel-client/src/main/java/com/alibaba/arthas/tunnel/client/ProxyClient.java - call the helper instead of the hard-coded constant.
• tunnel-client/pom.xml - add JUnit test scope (matches tunnel-common).

Tests Added

tunnel-client/src/test/java/com/taobao/arthas/common/ArthasConstantsTest.java covers:

• Defaults to MAX_HTTP_CONTENT_LENGTH when the property is unset.
• Honours a configured value (100 MB).
• Falls back to default on non-numeric input.
• Falls back to default on zero/negative input.

mvn -pl common,tunnel-client -am clean test passes locally (4 new tests, 0 failures).

Impact

• Backward compatible - default behavior unchanged.
• Scoped to the tunnel-client HTTP aggregator path identified in the issue; does not alter the MAX_HTTP_CONTENT_LENGTH constant or the many other call sites that depend on its current value.
• Operators downloading large JFR recordings can opt in by setting the new system property on the agent JVM.

Closes #3034