[MCP Stateless] Slice B-3: Mock stateless server + integration tests for the stateless path

[Lexus2016]

Parent: #1287 (Slice B decomposition)

This is Slice B-3 of the MCP 2026-07-28 stateless migration. Depends on #1511 (B-1) and #1512 (B-2).

Scope (this issue — ≤200 lines)

Add a mock stateless MCP server and comprehensive integration tests that validate the stateless request path end-to-end. This closes the verification gap — Slices B-1 and B-2 add the code paths, this issue proves they work.

Specific changes

1. Mock stateless server fixture: a test helper that simulates a stateless MCP server (responds to server/discover, does NOT implement initialize/initialized, routes via _meta inline). Can be a lightweight unittest.mock or asyncio test server.

2. Test: capability detection — verify _detect_stateless_support() correctly identifies stateless vs stateful servers.

3. Test: synthesize_capabilities flow — verify synthesize_capabilities() produces a valid InitializeResult from a server/discover response that downstream consumers accept.

4. Test: stateless call_tool — verify the hot path (line ~4263) routes correctly in stateless mode and returns tool results.

5. Test: flag-off regression — verify the existing stateful path is 100% unchanged when HERMES_MCP_STATELESS is not set.

6. Test: session-id absence — verify Mcp-Session-Id header is NOT sent in stateless mode.

What this issue does NOT do

• No production code changes (all test/fixtures only)
• Depends on B-1 ([MCP Stateless] Slice B-1: Capability-detection flag + synthesize_capabilities() #1511) and B-2 ([MCP Stateless] Slice B-2: Stateless request routing — skip initialize, add _meta inline + server/discover #1512) being merged

Success criteria

• Mock stateless server fixture created
• Capability detection test passes
• synthesize_capabilities flow test passes
• Stateless call_tool integration test passes
• Flag-off regression test confirms zero behavioral change
• Session-id absence test passes
• All tests pass with pytest tests/ -x -q
• Fits ≤200-line self-merge cap

Dependencies

• [MCP Stateless] Slice B-1: Capability-detection flag + synthesize_capabilities() #1511 (B-1) must land first
• [MCP Stateless] Slice B-2: Stateless request routing — skip initialize, add _meta inline + server/discover #1512 (B-2) must land first

Audit reference

Full call-site map: docs/mcp/2026-07-28-stateless-migration-audit.md