Support MCP spec 2026-07-28 across client and servers

[RhysSullivan]

Executor speaks MCP spec 2026-07-28 in both directions. Legacy clients are untouched on their existing code paths — this PR is purely additive routing (part 1 of 2; the follow-up consolidates legacy serving onto SDK v2 and deletes the agents patch).

• Outbound client (packages/plugins/mcp): SDK v2; streamable-HTTP connections auto-negotiate the era (server/discover probe, legacy fallback); typed v2 error classification; shape probe handles modern-only servers. Fixes the reported failure where executor sent 2025-11-25 to a modern-only server and was rejected.
• Tool server (packages/hosts/mcp): SDK-agnostic core with two thin assemblies — v1 byte-identical for existing paths, new v2 assembly for stateless modern serving. Native elicitation = input_required + HMAC-signed requestState (principal+method bound, TTL'd) over the existing pause/resume engine. MCP Apps helpers vendored wire-identical pending upstream ext-apps v2 ([codex] Put typed base URL first in combobox #702).
• Neutral hosts (selfhost, local): era-routed after auth; modern → cached stateless handlers; legacy → existing sessionful store unchanged. CORS preflights echo requested headers for dynamic Mcp-Param-<name>.
• Cloudflare hosts: modern reads worker-side (no DO); modern tools/call routes into DOs for execution affinity via the executionId-keyed owner directory; legacy stays on the untouched McpAgent path. Same cross-bearer 403 contract as legacy.

Evidence: full e2e suites green (legacy non-regression); modern support proven live with a 2026-07-28-pinned client (discover → tools/list → execute); the pre-migration client reproduces the reported error byte-for-byte against a legacy: "reject" server while this client connects.

Deployment prerequisite (before merge): both workers need the shared continuation-signing secret (≥32 bytes), visible to worker + session DOs:

wrangler secret put MCP_REQUEST_STATE_KEY

Follow-up PR: #1611 (consolidation — deletes the 911-line agents patch, McpAgent, and the v1 assembly).

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✅ Deployment successful! View logs	executor-marketing	3220892	Commit Preview URL Branch Preview URL	Aug 16 2026, 12:35 PM

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commit: 3220892

[RhysSullivan]

Re-split by protocol role per review preference: client = #1591, server = #1611.