E2E Tests Core (proxy) broken on all PRs: batch request now 400s in TimeStreamableHttpMcpServer

[samuv]

Summary

The E2E Tests Core (proxy) shard is failing deterministically on every open PR, including PRs that don't touch proxy/transport code at all:

• Add project-level skills lock file schema package #5892 (lockfile schema package only) — failed twice, including a rerun
• Add SkillLockService interface and managed install flag #5893, Record project-scope skill installs in the lock file #5894, Add thv skill sync: restore project skills from the lock file #5895, Add thv skill upgrade: re-resolve pinned skills to newer content #5896, Add typed exit codes and confirmation gate to skill sync/upgrade #5897 (skills-lock stack)
• Add untrusted mode: MCP servers that never possess credentials #5914 (untrusted MCP egress broker — unrelated area)

Failing spec: TimeStreamableHttpMcpServer > when starting the time server with streamable-http proxy > should respond to a single get_current_time request and a batch request (test/e2e/stdio_proxy_over_streamable_http_mcp_server_test.go:42)

Assertion: the batch request part now returns HTTP 400 where 200 is expected.

Likely cause

This looks like fallout from the recent stateless-proxy work on main (e.g. #5884, "Serve MCP 2026-07-28 stateless requests through the transparent proxy") — the MCP 2026-07-28 revision removed JSON-RPC batching, so the proxy rejecting a batch with 400 may be intentional spec-conformant behavior with a test that wasn't updated, or an unintended regression for older-protocol clients.

Either way the spec and the proxy currently disagree, and every PR's required check is red because of it.

Impact

All open PRs show mergeState=UNSTABLE on this shard regardless of their content. Example failing job: https://github.com/stacklok/toolhive/actions/runs/29992236950/job/89174538909

[samuv]

Root cause confirmed: the 400 is intentional — #5931 deliberately rejects JSON-RPC batches at the proxy, but the E2E spec still asserted the old 200-with-results behavior. Fix (test-only): #5935.