Update standalone-SSE test for filtered-tool error

[JAORMX]

Summary

Why: main's required Tests / Test Go Code check is currently red on every PR. TestStandaloneSSE_ListChangedRefiltersThroughExistingMiddleware (pkg/transport/proxy/streamable/dispatcher_standalone_sse_integration_test.go:468) asserts that a tools/call for a filtered tool returns HTTP 400 — but #5944 changed the tool-call filter to return a JSON-RPC error over HTTP 200 (a filtered tool is now indistinguishable from a nonexistent one). The test was added by #5934; #5934 and #5944 landed close together, so each was green in isolation but their combination is a semantic merge collision that leaves the unit-test suite deterministically failing on main.

What:

• Update step (3) of the test to assert the current behavior: HTTP 200 with a JSON-RPC error body containing "tool not found". The tool is still blocked (never forwarded to the backend); only the response envelope changed.

Verified locally: the test failed deterministically before this change and passes after; task lint clean.

Type of change

• Bug fix

Test plan

• Unit tests — go test -race -run TestStandaloneSSE_ListChangedRefiltersThroughExistingMiddleware ./pkg/transport/proxy/streamable/ passes (failed before the change).
• Linting — task lint 0 issues.

Does this introduce a user-facing change?

No. Test-only change aligning an assertion with the behavior shipped in #5944.

Special notes for reviewers

• This is a test-only fix for a main-branch regression; no production code changes. It unblocks the required Tests check for all open PRs.
• Root cause is the Route server-to-client MCP messages per session #5934 (test) × Return JSON-RPC error for filtered tool calls #5944 (behavior) merge interaction — neither PR's own CI saw the combination.

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.83%. Comparing base (c007e6a) to head (c1eabb0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5958      +/-   ##
==========================================
+ Coverage   71.80%   71.83%   +0.03%     
==========================================
  Files         708      708              
  Lines       72767    72798      +31     
==========================================
+ Hits        52250    52296      +46     
+ Misses      16778    16764      -14     
+ Partials     3739     3738       -1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.