feat(mcp): stateless transport shim — Slice B-1 (#1511)

[Lexus2016]

Summary

Implements Slice B-1 of the MCP 2026-07-28 stateless transport migration (issue #1511, parent #1287).

Changes

• HERMES_MCP_STATELESS env flag — default OFF, zero behavioral change unless enabled
• _detect_stateless_support(config) — reads env flag + per-server stateless: true config key; both must be truthy
• synthesize_capabilities() — constructs an InitializeResult-shaped SimpleNamespace with all three capability families (tools, resources, prompts) so existing capability gates work without a live session
• _initialize_or_synthesize() — helper method on MCPServerTask that skips session.initialize() in stateless mode and returns synthesized caps; delegates to the real handshake otherwise
• All 4 transport call sites wired — stdio, SSE, streamable_http new API, streamablehttp deprecated API

Design

• Feature flag defaults OFF → no behavioral change for any existing deployment
• Per-server opt-in via stateless: true in config.yaml (requires global env flag too)
• Synthesized caps object is permissive (all capabilities ON) — a future slice may probe .well-known/mcp to narrow the set
• Existing capability gates (_advertises_tools(), _select_utility_schemas()) are capability-source-agnostic and work unchanged with synthesized caps

Test plan

• 14 unit tests (tests/tools/test_mcp_stateless_b1.py) — all pass
• Tests cover: env flag gating, per-server config opt-in/out, synthesized caps shape, _initialize_or_synthesize() both paths
• No existing MCP tests broken (flag defaults OFF)

Scope

This is Slice B-1 only. B-2 (stateless RPC adapter, header injection, per-call sessions) and B-3 (mock tests) are separate issues (#1512, #1513).

Closes #1511

[Lexus2016]

Superseded by #1519 (newer implementation of the same issue #1511 — MCP stateless B-1). #1519 is cleaner (methods on MCPServerTask vs module-level functions) and #1518 has a spurious deletion of 'return reason' at the SSE reconnect path. Closing this duplicate to avoid confusion.