fix(client): respond when request handlers complete empty

[1fanwang]

An asynchronous sampling or elicitation handler can complete without a value.
The client then sends no JSON-RPC response, leaving the server blocked until
its request timeout. The client now returns internal error -32603 with the
original request ID.

Motivation and Context

JSON-RPC requires one response for every request with an ID. This converts an
empty client handler completion into an explicit error while preserving normal
results and handler failures.

Closes #1124

The equivalent server-side paths are covered by
#1099.

How Has This Been Tested?

The regression fails on clean main, passes with this change, and the live MCP
2025-11-25 client elicitation scenario still passes.

git worktree add --detach ../java-sdk-main 39c225e46c51311eaf82192f15b950e421580fb5
git diff 39c225e46c51311eaf82192f15b950e421580fb5..HEAD -- \
  mcp-core/src/test/java/io/modelcontextprotocol/spec/McpClientSessionTests.java \
  | git -C ../java-sdk-main apply -
(cd ../java-sdk-main && ./mvnw -pl mcp-core \
  -Dtest=McpClientSessionTests#testEmptyRequestHandlerSendsErrorResponse test)

./mvnw -pl mcp-core \
  -Dtest=McpClientSessionTests#testEmptyRequestHandlerSendsErrorResponse test
./mvnw -pl mcp-core test
./mvnw -pl conformance-tests/client-jdk-http-client -am package -DskipTests
npx -y @modelcontextprotocol/conformance@0.2.0-alpha.11 client \
  --spec-version 2025-11-25 \
  --command "java -jar conformance-tests/client-jdk-http-client/target/client-jdk-http-client-2.1.0-SNAPSHOT.jar" \
  --scenario elicitation-sep1034-client-defaults \
  --expected-failures ./conformance-tests/conformance-baseline.yml

Raw logs

Before:
Tests run: 1, Failures: 1, Errors: 0, Skipped: 0
Expecting actual not to be null
BUILD FAILURE

After:
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

MCP core:
Tests run: 400, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

Client conformance:
Passed: 5/5, 0 failed, 0 warnings
OVERALL: PASSED

Breaking Changes

None.

Types of changes

• Bug fix

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed