feat(mcp): support the 2026-07-28 protocol

[bokelley]

What

• Add MCP 2026-07-28 negotiation and serving through the official MCP v2 beta.4 packages.
• Preserve the v1 client/server path for legacy peers and experimental 2025 Tasks compatibility.
• Negotiate modern MCP in high-level discovery, tool calls, and OAuth flows without replaying ambiguous failures.
• Add bounded connection/legacy-classification caches, redirect credential protection, per-request tracing, request-size/media-type guards, and Host/Origin validation.
• Preserve per-principal tool visibility across legacy and modern serving, including comply_test_controller.
• Close signed-request batch bypass and public verifier-detail leakage found during expert review.

Breaking changes

• Raise the minimum Node.js version from 18 to 20, required by MCP SDK v2.
• Host/Origin validation applies before protocol-era classification. Public deployments must configure publicUrl or explicit allowedHosts/allowedOrigins; reverse proxies that rewrite Host must include the upstream hostname.
• Raw v1 McpServer instances remain legacy-only. Use framework AdcpServer factories for dual-era serving.

Validation

• npm run build:lib
• modern/auth/timeout regressions: 27 passed
• signed-request regressions: 29 passed
• seller adapter/conformance path: 5 passed
• broader server/protocol suite: 84/85 passed before fixing the sole timeout-code assertion; the corrected focused suite then passed
• mcp-spec-check: grade A, ready for 2026-07-28 (6 pass, 0 fail, 2 not-applicable skips)
• npm audit --omit=dev: 0 vulnerabilities
• DX, protocol, code, and security expert reviews converged with no remaining actionable findings

Includes a major changeset for @adcp/sdk.