fix(java): stop logging the legacy 'connect' probe failure as a warning

[rinceyuan]

Summary

CopilotClient.verifyProtocolVersion probes the connect RPC and falls back to ping when the server does not implement it. That fallback works, but JsonRpcClient.invoke logged every failed request at WARNING with a full stack trace, so the recovered probe printed this on a plain tutorial run (JUL sends WARNING to the console by default):

WARNUNG: JsonRpc.invoke JSON-RPC request finished. Elapsed=PT0.793S, Method=connect, RequestId=1, Status=Failed
java.util.concurrent.CompletionException: com.github.copilot.JsonRpcException: Unhandled method connect

Change

• JsonRpcClient.invoke gets an internal (package-private) overload that takes the level used for failure logging; the existing 3-arg overload still defaults to WARNING.
• The protocol-negotiation connect probe passes Level.FINE, since CopilotClient already catches and recovers from that exact failure.

No public API change - JsonRpcClient is package-private. Genuinely unexpected RPC failures still log at WARNING.

Tests

Two cases added to JsonRpcClientTest, both driving a real socket pair and a -32601 Unhandled method connect error response while capturing the JUL records:

• testInvokeLogsFailureAtWarningByDefault - unexpected failures are still WARNING.
• testInvokeHonorsCustomFailureLogLevel - an expected/recovered failure produces nothing at WARNING or above.

Verified testInvokeHonorsCustomFailureLogLevel fails when the level argument is ignored.

mvnw.cmd test -Dtest=JsonRpcClientTest -Denforcer.skip=true -Pskip-test-harness   # 25/25 pass
mvnw.cmd spotless:check -DspotlessFiles=... ; mvnw.cmd checkstyle:check           # clean

Fixes #2291.

[Copilot]

Pull request overview

Adds configurable RPC failure logging to suppress warnings for legacy connect probes.

Changes:

• Adds an internal failure-log-level overload.
• Uses FINE for protocol negotiation.
• Adds logging-level tests.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
CopilotClient.java	Downgrades connect probe failures.
JsonRpcClient.java	Adds configurable failure logging.
JsonRpcClientTest.java	Tests default and custom levels.

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[rinceyuan]

Good catch - fixed.

The level is no longer decided up front. JsonRpcClient.invoke now takes a Predicate<Throwable> describing the failure the caller recovers from, evaluates it against the unwrapped cause inside exceptionally, and logs FINE only on a match. Everything else - -32603, deserialization failures, I/O failures, timeouts - still logs at WARNING.

The probe passes exactly the condition verifyProtocolVersion recovers from, so the two can no longer drift:

cause -> cause instanceof JsonRpcException rpcEx && isUnsupportedConnectMethod(rpcEx)

Added testInvokeKeepsWarningForUnexpectedFailure, which drives a -32603 through the same predicate and asserts a WARNING is still emitted. It fails against the previous revision.

mvnw test -Dtest=JsonRpcClientTest 26/26, spotless and checkstyle clean.